Reactivity of Cyclopentadienyl Molybdenum Compounds towards

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Article pubs.acs.org/IC

Reactivity of Cyclopentadienyl Molybdenum Compounds towards Formic Acid: Structural Characterization of CpMo(PMe3)(CO)2H, CpMo(PMe3)2(CO)H, [CpMo(μ-O)(μ‑O2CH)]2, and [Cp*Mo(μO)(μ‑O2CH)]2 Michelle C. Neary and Gerard Parkin* Department of Chemistry, Columbia University, New York, New York 10027, United States S Supporting Information *

ABSTRACT: The molecular structures of CpMo(PMe3)(CO)2H and CpMo(PMe3)2(CO)H have been determined by X-ray diffraction, thereby revealing four-legged piano-stool structures in which the hydride ligand is trans to CO. However, in view of the different nature of the four basal ligands, the geometries of CpMo(PMe3)(CO)2H and CpMo(PMe3)2(CO)H deviate from that of an idealized four-legged piano stool, such that the two ligands that are orthogonal to the trans H−Mo−CO moiety are displaced towards the hydride ligand. While CpRMo(PMe3)3−x(CO)xH (CpR = Cp, Cp*; x = 1, 2, 3) are catalysts for the release of H2 from formic acid, the carbonyl derivatives, CpRMo(CO)3H, are also observed to form dinuclear formate compounds, namely, [CpRMo(μ-O)(μO2CH)]2. The nature of the Mo···Mo interactions in [CpMo(μ-O)(μ-O2CH)]2 and [Cp*Mo(μ-O)(μ-O2CH)]2 have been addressed computationally. In this regard, the two highest occupied molecular orbitals of [CpMo(μ-O)(μ-O2CH)]2 correspond to metal-based δ* (HOMO) and σ (HOMO−1) orbitals. The σ2δ*2 configuration thus corresponds to a formal direct Mo−Mo bond order of zero. The preferential occupation of the δ* orbital rather than the δ orbital is a consequence of the interaction of the latter orbital with p orbitals of the bridging oxo ligands. In essence, lone-pair donation from oxygen increases the electron count so that the molybdenum centers can achieve an 18-electron configuration without the existence of a Mo−Mo bond, whereas a MoMo double bond is required in the absence of lone-pair donation.

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INTRODUCTION The utilization of fuels that are both more sustainable than those derived from fossil feedstocks and have a reduced environmental impact is essential in view of the growing demand for energy.1 In this regard, hydrogen is considered to be a promising alternative because it is renewable2 and its combustion or use in a fuel cell releases water as the only byproduct.3 However, despite these benefits, current storage and transportation techniques are inadequate for the effective use of hydrogen as a fuel.4 Therefore, much effort has been directed towards developing methods for delivering hydrogen on demand,5 with formic acid having received considerable attention because it is a liquid at room temperature and is easy to transport.6−8 The use of formic acid to generate hydrogen on demand, however, requires efficient catalysis, and both heterogeneous9,10 and homogeneous11−22 systems have been investigated, with a particular emphasis being the discovery of catalysts that feature earth-abundant nonprecious metals.16−20 For example, we recently demonstrated that the cyclopentadienyl compounds CpRMo(PMe3)3−x(CO)xH (CpR = Cp, Cp*; x = 1, 2, 3) are capable of serving as catalysts for the release of H2 from formic acid (Scheme 1).17 Here we report the molecular structures of some of these derivatives and the molybdenum products that result from the reactivity of © XXXX American Chemical Society

Scheme 1. Dehydrogenation of HCO2H

CpRMo(CO)3H towards formic acid, namely, [CpRMo(μO)(μ-O2CH)]2.

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RESULTS AND DISCUSSION 1. Molecular Structures of CpMo(PMe3)(CO)2H and CpMo(PMe3)2(CO)H. An interesting feature of the catalytic activity of CpRMo(PMe3)3−x(CO)xH is that it is not a monotonic function of the number of CO or PMe3 ligands. Specifically, the activity is greatest for the compounds with two PMe3 ligands, i.e., CpRMo(PMe3)2(CO)H.17 To complement our previous reactivity study, we have analyzed the molecular structures of CpMo(PMe3)3−x(CO)xH by X-ray diffraction. The molecular structures of CpMo(PMe3)(CO)2H (Figure 1) and CpMo(PMe 3 ) 2 (CO)H (Figure 2) have been determined by X-ray diffraction, and both compounds exhibit Received: October 26, 2016

A

DOI: 10.1021/acs.inorgchem.6b02606 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Possible isomeric structures of CpMo(PMe3)(CO)2H (top) and CpMo(PMe3)2(CO)H (bottom). In both cases, the observed structure in the solid state has a trans arrangement of the hydride and CO ligands.

Figure 1. Molecular structure of CpMo(PMe3)(CO)2H (hydrogen atoms bound to carbon have been omitted for clarity).

CpMo(PMe3)2(CO)H.27b The structure assigned for CpMo(PMe3)2(CO)H is, nevertheless, in accord with the solid-state structure reported here, in which the hydride and carbonyl ligands are trans to each other.27b Selected metrical data for CpRMo(PR3)3−x(CO)xH (Figure 4) are compared in Table 1.30−34 As would be anticipated given

Figure 4. Angles of interest for CpRMoL3H (L = CO, PR3) compounds: ligand labeling scheme (left); Newman projection down the Cp−Mo axis displaying the cis-L−Mo−L and cis-L−Mo− H angles (right).

Figure 2. Molecular structure of CpMo(PMe3)2(CO)H (hydrogen atoms bound to carbon have been omitted for clarity).

the different nature of the four basal ligands, the geometries deviate from that of an idealized four-legged piano stool. Specifically, the two ligands that are orthogonal to the trans H− Mo−CO moiety are displaced towards the hydride ligand, as illustrated in Figure 5. The four angles between pairs of cis ligands are therefore unequal, as are the angles between pairs of trans ligands. For example, the cis bond angles involving the hydride ligand (i.e., L1−Mo−H and L3−Mo−H), which range from 61.3° to 69.4°, are much smaller than those involving the ligand trans to the hydride (i.e., L1−Mo−L2 and L2−Mo−L3), which range from 77.7° to 88.2°. 2. Formation of Bridging Formate Compounds: Structure and Bonding in [CpMo(μ-O)(μ-O2CH)]2 and [Cp*Mo(μ-O)(μ-O2CH)]2. The activities of the aforementioned catalytic systems decrease over the course of the transformation, and analysis by 1H NMR spectroscopy and singlecrystal X-ray diffraction demonstrates that the reaction with CpMo(CO)3H is accompanied by the formation of dinuclear compounds, namely, [CpMo(CO)3]2 and [CpMo(μ-O)(μO 2 CH)] 2 ; 35,36 [Cp*Mo(CO) 3 ] 2 and [Cp*Mo(μ-O)(μO2CH)]2 are likewise formed from Cp*Mo(CO)3H37 in the presence of formic acid. The formation of [CpMo(CO)3]2 is not unexpected since CpMo(CO)3H has been demonstrated to be thermally unstable with respect to [CpMo(CO)3]2 and H2.24a,38 Although the bridging formate compound [CpMo(μO)(μ-O2CH)]2 has not been previously reported, it may also

four-legged piano-stool structures23 similar to that of the parent tricarbonyl compound, CpMo(CO)3H.24,25 However, whereas only one isomer is possible for the tricarbonyl compound, two isomers are possible for CpMo(PMe3)(CO)2H and CpMo(PMe3)2(CO)H. These isomers may be differentiated according to whether the hydride ligand is trans to CO or trans to PMe3, as illustrated in Figure 3.26 In this regard, both CpMo(PMe3)(CO)2H and CpMo(PMe3)2(CO)H exhibit a trans arrangement of the hydride and CO ligands in the solid state. NMR spectroscopic studies, however, have previously shown that, in solution, CpMo(PMe3)(CO)2H exists as a mixture of isomers that exchange rapidly on the NMR time scale; the isomer with the hydride ligand trans to CO is, nevertheless, more thermodynamically stable.27−29 NMR spectroscopic studies also indicate that the pentabenzylcyclopentadienyl compound, CpBzMo(PMe3)(CO)2H, exists as a mixture of isomers in solution, but in contrast to CpMo(PMe3)(CO)2H, the isomer in which the hydride ligand is trans to PMe3 is observed in the solid state;30 the triphenylphosphine derivative, CpBzMo(PPh3)(CO)2H, however, adopts a trans arrangement of the hydride and CO ligands in the solid state,30 akin to that of CpMo(PMe3)(CO)2H. Interestingly, whereas the monophosphine compound CpMo(PMe3)(CO)2H exists as two isomers in solution, only one isomer is observed for the bisphosphine derivative, B

DOI: 10.1021/acs.inorgchem.6b02606 Inorg. Chem. XXXX, XXX, XXX−XXX

a

79.67 88.19 81.60 79.04 78.34 81.86 80.36 82.81 82.23 80.50

L1−Mo−L2/deg

L1−Mo−L3/deg 101.85 101.46 105.88 107.30 103.63 102.24 108.75 98.52 102.78 116.09

L2−Mo−L3/deg 82.12 80.65 81.73 79.47 83.46 80.91 83.56 77.72 81.19 80.51

66.65 − 66.94 65.94 64.14 67.10 69.82 65.87 65.71 65.15

L1−Mo−H/deg

L3−Mo−H/deg 61.26 − 63.38 63.66 63.62 63.56 69.54 67.42 62.53 65.85

L2−Mo−H/deg 121.32 − 121.86 115.74 119.72 124.46 129.50 127.56 122.20 110.26

The hydride ligand was not located. bIsomer with hydride trans to CO. cIsomer with hydride trans to PMe3. dTwo crystallographically independent molecules.

Cp*Mo[P(OMe)(NMeCH2)2](CO)2Hb CpBzMo(PMe3)(CO)2Hc CpBzMo(PPh3)(CO)2Hb CpMo(PMe3)2(CO)Hb

CpMo(CO)3H Cp*Mo(CO)3Ha CpBzMo(CO)3H CpMo(PMe3)(CO)2Hb CpMo(PPri3)(CO)2Hb,d

compound

Table 1. Selected Bond Length and Bond Angle Data for CpRMoL3H Compounds (L = CO, PR3) 25 31 32 this work 33

1.693 − 1.606 1.670 1.643 1.650 1.693 1.778 1.812 1.694

34 30 30 this work

ref

Mo−H/Å

Inorganic Chemistry Article

Figure 5. Displacement of the two ligands that are orthogonal to the trans-H−Mo−CO moiety towards the hydride ligand. Coordinates for CpMo(CO)3H were taken from ref 25.

be independently synthesized by the reduction of [CpMo(O)(μ-O)]239 with zinc in the presence of formic acid (Scheme 2) using a procedure similar to that employed for the acetate derivative, [Cp*Mo(μ-O)(μ-O2CMe)]2.40

Scheme 2. Synthesis of [CpMo(μ-O)(μ-O2CH)]2 and [Cp*Mo(μ-O)(μ-O2CH)]2

The molecular structures of [CpMo(μ-O)(μ-O2CH)]2 (Figure 6) and [Cp*Mo(μ-O)(μ-O2CH)]2 (Figure 7) have been determined by X-ray diffraction, and the Mo···Mo distances are listed in Table 2. While dinuclear molybdenum

Figure 6. Molecular structure of [CpMo(μ-O)(μ-O2CH)]2 (Cp hydrogen atoms have been omitted for clarity).

C

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we have addressed the nature of the Mo···Mo interaction in [CpMo(μ-O)(μ-O2CH)]2 and [Cp*Mo(μ-O)(μ-O2CH)]2 computationally. Density functional theory (DFT, B3LYP) geometryoptimized structures for [CpMo(μ-O)(μ-O 2CH)] 2 and [Cp*Mo(μ-O)(μ-O2CH)]2 (Figure 8) reproduce well the experimentally determined structures, as summarized in Table 4. Pertinent molecular orbitals of [CpMo(μ-O)(μ-O2CH)]2 are illustrated in Figure 9, which shows that, interestingly, the two highest occupied orbitals correspond to metal-based δ* (HOMO) and σ (HOMO−1) orbitals.60 The σ2δ*2 configuration thus corresponds to a formal direct Mo−Mo bond order of zero.61−63 The preferential occupation of the δ* orbital rather than the δ orbital is a consequence of the interaction of the latter with p orbitals of the bridging oxo ligands, which serves to raise its energy above that of the δ* orbital. In this regard, previous studies have demonstrated that, depending on the nature of bridging ligands, the δ* combination may be lower in energy than the δ combination.44,64−71 As a result of this energy inversion, the formal metal−metal bond order may often be lower than otherwise predicted by assuming that the energies of the molecular orbitals derived by the interaction of metal−metal d orbitals increase in the sequence σ < π < δ < δ* < π* < σ*. A structure−bonding representation72 of [CpMo(μO)(μ-O2CH)]2 that is consistent with this molecular orbital description is shown in Figure 10 (left). Specifically, lone-pair donation73 from oxygen to molybdenum increases the electron count such that the molybdenum centers can achieve an 18electron configuration without the existence of a Mo−Mo bond. In contrast, the absence of lone-pair donation from the oxygen atoms would require a MoMo double bond to achieve an 18-electron configuration for each molybdenum atom, as illustrated in Figure 10 (right). In support of the role of the oxo ligands in inverting the energies of the δ and δ* orbitals, calculations on the [CpMo(μO2CH)]2 fragment, in which the oxo ligands have been removed, indicate a σ2δ2π2π2 configuration,60 which corresponds to a formal quadruple bond (Figure 11). A qualitative molecular orbital diagram that illustrates how the interaction with the two oxo ligands serves to reduce the formal Mo−Mo bond order to zero is presented in Figure 12. Specifically, the π and δ orbitals of [CpMo(μ-O2CH)]2 interact sufficiently strongly with appropriate combinations of oxygen p orbitals that they are raised above the δ* orbital, which thereby becomes the HOMO. In contrast to the formal bond order of zero for [CpMo(μO)(μ-O2CH)]2 and [Cp*Mo(μ-O)(μ-O2CH)]2, the oxo compound Mo2(DXylF2,6)2(O2CMe)2(μ-O)2, which features two N,N′-di(2,6-xylylformamidate) ligands, is predicted to have a σ2π2 double bond and consequently has a much shorter

Figure 7. Molecular structure of [Cp*Mo(μ-O)(μ-O2CH)]2 (Cp* hydrogen atoms have been omitted for clarity).

compounds with bridging formate ligands are well-known, there are no examples listed in the Cambridge Structural Database (CSD)41 that also feature cyclopentadienyl ligands. Nevertheless, there are several examples of dinuclear cyclopentadienyl molybdenum compounds that incorporate other carboxylate ligands, as illustrated in Table 2.42−45 In addition to featuring cyclopentadienyl ligands, [CpMo(μO)(μ-O2CH)]2 and [Cp*Mo(μ-O)(μ-O2CH)]2 are also noteworthy because there are relatively few examples of dinuclear molybdenum formate complexes that possess bridging oxo ligands. Nevertheless, some examples with both single and double oxo bridges are known (Table 3), and a common feature is that the Mo···Mo distances for the compounds with two oxo bridges are shorter than those for the compounds with one oxo bridge. Correspondingly, the bond angles at oxygen in those compounds with two oxo bridges are acute, whereas the bond angles for those with one oxo bridge are significantly greater than 90° (Table 3).46−48 The large variation in the Mo···Mo distance in the molybdenum oxo formate compounds (Table 3) is in accord with the extensive literature on dinuclear molybdenum compounds, for which the Mo···Mo distances are known to span a very wide range. For example, the unsupported Mo−Mo distances in [CpMo(CO)3]2 [3.235(1) Å]49,50 and [Cp*Mo(CO)3]2 [3.281(1) Å]51 are more than 1 Å longer than those in compounds with Mo≣Mo quadruple bonds, which are typically in the range 2.06 to 2.17 Å,52 as illustrated by the tetraformate complex Mo2(μ-O2CH)4 [2.091(2) Å].53,54 While distances between pairs of atoms may often correlate with bond order, the presence of bridging ligands complicates the issue because the steric and electronic requirements of the bridging group may cause the metal−metal distance to be either shorter or longer than that in the absence of the bridge.55−59 Therefore,

Table 2. Mo···Mo Distances in Dinuclear {[CpRMo]2(μ-O2CR)} Compounds

[CpMo(μ-O)(μ-O2CH)]2 [Cp*Mo(μ-O)(μ-O2CH)]2 [Cp*Mo(μ-O)(μ-O2CMe)]2 [Cp*Mo]2(μ-O2CCF3)(μ-SMe)3 {[Cp*Mo]2(μ-O2CPh)(μ-PCy2)(μ-CPh)}[BF4] [Cp*MoCl]2(μ-O)(μ-O2COH)(μ-Cl) [Cp*Mo(μ-O2CMe)]2(μ-PMe2)(μ-Me) D

d(Mo···Mo)/Å

ref

2.5588(14) 2.554(2) 2.5524(3) 2.709(1) 2.576(1) 2.799(4) 2.8447(5)

this work this work 40 42 43 44 45 DOI: 10.1021/acs.inorgchem.6b02606 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 3. Mo···Mo Distances and Mo−O−Mo Angles in Dinuclear {Mo2(μ-O2CH)x(μ-O)y} Compounds d(Mo···Mo)/Å [CpMo(μ-O)(μ-O2CH)]2 [Cp*Mo(μ-O)(μ-O2CH)]2

Two μ-O Bridges 2.5588(14) 2.554(2)

{[Mo(O)(κ1-O2CH)2]2(μ-O2CH)(μ-O)2}[NH4]3

2.549(1)

{[Mo(O)Cl(py)]2(μ-O)2(μ-O2CH)}[(py)2H]

2.5483(6)

{[Mo(O)Br(py)]2(μ-O)2(μ-O2CH)}[(py)H]

2.5662(5)

{[Mo(O)Cl(py)]2(μ-O)2(μ-O2CH)}[(py)H]

2.5639(4)

{[Mo(O)Br2]2(μ-O)2(μ-O2CH)}[(py)H]3 {[Mo(O)(η2-O2)](μ-O)(μ-O2CH)]2}K {[Mo(O)(η2-O2)](μ-O)(μ-O2CH)]2}Rb {[Mo(O)(η2-O2)](μ-O2CH)(μ-O)]2}[NH4]

2.5728(5) One μ-O Bridge 3.707 3.718 3.723

Mo−O−Mo/deg

ref

83.33(10) 83.1(3) 82.8(3) 82.4(1) 82.49 82.48(12) 82.78(12) 81.98(11) 83.23(11) 82.93(8) 82.32(8) 82.66(10)

this work this work

148.49(9) 150.14(16) 149.10(8)

46 47 47 47 47 48 48 48

(NRT) natural bond order,79,80 as summarized in Table 5.81 While the Mo−Mo bond orders predicted by the different methods vary, all of the methods result in Mo−Mo bond orders for [CpMo(μ-O)(μ-O2CH)]2 that are substantially reduced from the corresponding values for related molecules that are devoid of bridging oxo ligands, namely, [CpMo(μ-O2CH)]2 and Mo2(μ-O2CH)4. It is also interesting to note that the NRT natural bond order of [CpMo(μ-O)(μ-O2CH)]2 is a sensitive function of the reference structures that are employed for the calculation. For example, Mo−Mo bond orders of 0.213 and 0.887 have been obtained. Despite this difference, the Mo−Mo bond orders are associated with comparable D(W) values (0.0125 and 0.0123, respectively),82 thereby indicating that the two solutions are indistinguishable with respect to their ability to model the electron density. Notwithstanding the inability of NRT to assign a definitive Mo−Mo bond in this case, the derived values are both less than unity and are therefore consistent with the σ2δ*2 configuration predicted by the molecular orbital analysis.

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SUMMARY CpMo(PMe3)(CO)2H and CpMo(PMe3)2(CO)H exhibit fourlegged piano-stool structures in which the hydride ligand is trans to CO. However, in view of the different nature of the four basal ligands, the geometries of CpMo(PMe3)(CO)2H and CpMo(PMe3)2(CO)H deviate from that of an idealized fourlegged piano stool, such that the two ligands that are orthogonal to the trans H−Mo−CO moiety are displaced towards the hydride ligand. While CpRMo(PMe3)3−x(CO)xH serve as catalysts for the release of H2 from formic acid, the tricarbonyl compounds, CpRMo(CO)3H, are also observed to react with formic acid to yield dinuclear formate derivatives, namely, [CpRMo(μ-O)(μ-O2CH)]2. Although the Mo−Mo bond lengths in [CpMo(μ-O)(μ-O2CH)]2 [2.5588(14) Å] and [CpMo(μ-O)(μ-O2CH)]2 [2.554(2) Å] are within the range previously reported for Mo−Mo bonding interactions, the presence of bridging ligands prevents the assignment of a bond order on the basis of bond length. The nature of the Mo···Mo interactions has therefore been addressed computationally, and analysis of the molecular orbitals indicates that the two highest occupied orbitals of [CpMo(μ-O)(μ-O2CH)]2 correspond to metal-based δ* (HOMO) and σ (HOMO−1) orbitals. The

Figure 8. DFT geometry-optimized structures of [CpMo(μ-O)(μO2CH)]2 and [Cp*Mo(μ-O)(μ-O2CH)]2. Hydrogen atoms on Cp and Cp* ligands are omitted for clarity.

average Mo−Mo distance of 2.306(2) Å.74 Similarly, the tungsten oxo compound (H2TMhpp)2[W(μ-O)(μ-TMhpp)Cl2]2 is also predicted to have a σ2π2 electronic configuration, which is in accord with the short W−W distance of 2.3318(8) Å.69,75 However, despite the fact that all of these compounds have two bridging oxo ligands in common, important distinctions are that Mo2(DXylF2,6)2(O2CMe)2(μ-O)2 and {[W(μ-O)(μ-TMhpp)Cl2]2}2− (i) exhibit a trans disposition of the oxo ligands and (ii) do not possess axially coordinated cyclopentadienyl ligands. Thus, the molecular orbital diagrams for these two different classes of compounds are not expected to be the same. In addition to analysis of the molecular orbitals, the nature of the interaction between the two molybdenum centers has been quantified by a variety of metrics, including the Wiberg index,76 the Mayer bond order,62,77,78 and the natural resonance theory E

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Table 4. Comparison of Experimental and Geometry-Optimized Structures of [CpMo(μ-O)(μ-O2CH)]2 and [Cp*Mo(μ-O)(μO2CH)]2 [CpMo(μ-O)(μ-O2CH)]2 d(Mo−Mo)/Å d(Mo−Ooxo)/Å

d(Mo−Oformate)/Å

[Cp*Mo(μ-O)(μ-O2CH)]2

exptl

calcd

exptl

calcd

2.5588(14) 1.923(2) 1.926(2) −a −a 2.156(3) 2.159(3) −a −a

2.583 1.934 1.934 1.934 1.934 2.203 2.204 2.203 2.204

2.554(2) 1.926(5) 1.931(5) −a −a 2.180(4) 2.183(5) −a −a

2.575 1.941 1.942 1.941 1.942 2.219 2.223 2.219 2.223

a

[CpMo(μ-O)(μ-O2CH)]2 resides on a crystallographic twofold axis, while [Cp*Mo(μ-O)(μ-O2CH)]2 resides on a crystallographic mirror plane; only crystallographically unique bond lengths are given.

Figure 9. Molecular orbitals for [CpMo(μ-O)(μ-O2CH)]2. The occupation of the σ (HOMO-1) and δ* (HOMO) orbitals corresponds to a formal bond order of zero. The δ orbital (LUMO) is higher in energy than the δ* orbital (HOMO) as a result of interaction with the oxo ligands.

Figure 11. Molecular orbitals for hypothetical [CpMo(μ-O2CH)]2 illustrating a formal Mo−Mo quadruple bond.

Figure 10. Structure−bonding representations of [CpMo(μ-O)(μO2CH)]2 with no Mo−Mo bond (left) and a MoMo double bond (right). In each case, only one possible resonance structure is shown.

ligands. In essence, lone-pair donation from oxygen increases the electron count so that the molybdenum centers can achieve an 18-electron configuration without the existence of a Mo− Mo bond, whereas a MoMo double bond is required in the absence of lone-pair donation. The bonding in [CpMo(μO)(μ-O2CH)]2 and [Cp*Mo(μ-O)(μ-O2CH)]2 can therefore

σ2δ*2 configuration thus corresponds to a formal direct Mo− Mo bond order of zero. The preferential occupation of the δ* orbital rather than the δ orbital is a consequence of the interaction of the latter with p orbitals of the bridging oxo F

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Figure 12. Qualitative molecular orbital diagram for [CpMo(μ-O)(μ-O2CH)]2 derived from the interaction of {[CpMo(μ-O2CH)]2} and two O ligands in C2v symmetry (the Mo−Mo vector is coincident with the y axis). The two occupied metal-based orbitals have σ and δ* character, resulting in a formal Mo−Mo bond order of zero. Note that the symmetry-adapted linear combinations (SALCs) of oxygen p orbitals are drawn perpendicular to the Mo−Mo vector. Also, for clarity, molecular orbitals are linked only to their principal components (i.e., the dimolybenum fragment or the oxo ligands).

Table 5. Comparisons of Mo−Mo Bond Orders for Dinuclear Molybdenum Compounds with Bridging Formate Ligands As Derived by Several Different Methods [CpMo(μ-O)(μ-O2CH)]2 [CpMo(μ-O2CH)]2b Mo2(μ-O2CH)4

d(Mo···Mo)/Å (calculated)

formal bond order

Wiberg bond index

Mayer bond order

NRT natural bond order

2.583 2.583 2.121

0 4 4

0.947 2.878 3.265

1.185 3.121 3.624

0.213, 0.887a 3.911 4.000

a

The derived bond order is a sensitive function of the reference structures employed for the calculation (see the text). bThe structure is identical to the [CpMo(μ-O2CH)]2 core of [CpMo(μ-O)(μ-O2CH)]2. G

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Inorganic Chemistry be conveniently represented by two resonance structures, one with and one without lone-pair donation from oxygen to molybdenum.

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Table 6. Crystal, Intensity Collection, and Refinement Data CpMo(PMe3)(CO)2H lattice monoclinic formula C10H15MoO2P formula weight 294.13 space group P21/c a/Å 8.4275(17) b/Å 12.152(2) c/Å 12.291(2) α/deg 90 β/deg 104.671(3) γ/deg 90 V/Å3 1217.7(4) Z 4 T/K 130(2) λ/Å 0.71073 ρcalcd/g cm−3 1.604 μMo Kα/mm−1 1.182 θmax/deg 32.928 no. of data collected 20362 no. of data 4360 no. of parameters 134 R1 [I > 2σ(I)] 0.0265 wR2 [I > 2σ(I)] 0.0597 R1 [all data] 0.0304 wR2 [all data] 0.0629 GOF 1.082 Rint 0.0316 [CpMo(μ-O)(μ-O2CH)]2

EXPERIMENTAL SECTION

General Considerations. All of the manipulations were performed using a combination of glovebox, high-vacuum, and Schlenk techniques under a nitrogen or argon atmosphere.83 Solvents were purified and degassed by standard procedures. NMR spectra were measured on Bruker 300 DRX and Bruker Avance 500 DMX spectrometers. 1H NMR spectra are reported in parts per million relative to SiMe4 (δ = 0) and were referenced internally with respect to the protio solvent impurity (δ = 7.16 for C6D5H and 7.26 for CDCl3).84 13C NMR spectra are reported in parts per million relative to SiMe4 (δ = 0) and were referenced internally with respect to the solvent (δ = 128.06 for C6D6).84 Coupling constants are reported in hertz. Infrared spectra were recorded on a PerkinElmer Spectrum Two spectrometer and are reported in reciprocal centimeters. Chemicals were obtained from Sigma-Aldrich (formic acid) and Cambridge Isotope Laboratories (H13CO2H) and used as supplied. CpMo(CO)3H,17b,24b,c,25 CpMo(PMe3)(CO)2H,17b,27 CpMo(PMe3)2(CO)H,17b,27b Cp*Mo(CO)3H,17b,37 and [CpMo(O)(μ-O)]239 were prepared by the literature methods. X-ray Structure Determinations. Crystals of CpMo(PMe3)(CO)2H and CpMo(PMe3)2(CO)H suitable for X-ray diffraction were obtained via slow evaporation of solutions in a 4:1 pentane/benzene mixture. X-ray diffraction data were collected on a Bruker Apex II diffractometer. Crystal data and data collection and refinement parameters are summarized in Table 6. The structures were solved using direct methods and standard difference map techniques and were refined by full-matrix least-squares procedures on F2 with SHELXTL (version 2014/7).85 Computational Details. Calculations were carried out using DFT as implemented in the Jaguar 8.9 (release 15)86 suite of ab initio quantum-chemistry programs. Geometry optimizations were performed with the B3LYP density functional using the 6-31G**++ (H, C, O, P) and LACVP (Mo) basis sets,87 which were also used to calculate Wiberg bond indices and Mayer bond orders. NRT calculations were performed with NBO 6.088 as implemented in the Jaguar suite of programs using the 6-31G**++ and LAV3P basis sets. Synthesis of [CpMo(μ-O)(μ-O2CH)]2. (a) A solution of formic acid (0.4 mL, 10 mmol) in MeOH/H2O (1:1 by volume, 8.0 mL) was transferred slowly via cannula to an ampule containing [CpMo(O)(μO)]2 (63 mg, 0.163 mmol) and zinc (639 mg, 9.78 mmol). The graybrown slurry was stirred under N2 for 4 days. The mixture was filtered via cannula, and the volatile components of the blue-green filtrate were removed in vacuo to afford a dark-green powder. The solid was extracted with CHCl3 (2 × 1 mL), and the volatile components were removed from the extract in vacuo to afford [CpMo(μ-O)(μ-O2CH)]2 as a dark-greenish-blue powder (7 mg, 11% yield). Anal. Calcd for [CpMoO(O2CH)]2: C, 32.45%; H, 2.72%. Found: C, 32.33%; H, 2.58%. 1H NMR (CDCl3): 6.68 [s, 10H, [(C5H5)MoO(O2CH)]2], 7.98 [s, 2H, [CpMoO(O2CH)]2]. IR (cm−1): 3120 (w), 3055 (w), 2952 (w), 2921 (w), 2856 (w), 1694 (w), 1633 (w), 1574 (w), 1545 (s), 1427 (m), 1345 (s), 1260 (w), 1113 (w), 1011 (m), 813 (s), 780 (m), 769 (m), 711 (m), 664 (s). (b) CpMo(CO)3H (5.0 mg, 0.020 mmol) was added to a solution of formic acid (7.7 μL, 0.20 mmol) in C6D6 (ca. 0.7 mL). The sample was heated to 100 °C and monitored by 1H NMR spectroscopy. After a period of 8 days, all of the formic acid was consumed, and hydrogen, methanol, methyl formate, CpMo(CO)3H, and [CpMo(CO)3]289 were observed by 1H NMR spectroscopy, while blue-red dichroic crystals of [CpMo(μ-O)(μO2CH)]2 were deposited on the side of the tube. An additional aliquot of formic acid (7.7 μL, 0.20 mmol) was added, and the sample was heated to 100 °C for 1 day, after which period crystals of [CpMo(μO)(μ-O2CH)]2 suitable for X-ray diffraction were isolated. Synthesis of [Cp*Mo(μ-O)(μ-O2CH)]2. Formic acid (10 μL, 0.27 mmol) was added to a solution of Cp*Mo(CO)3H (10 mg, 0.032 mmol) in C6D6 (ca. 0.7 mL). The mixture was heated at 100 °C, and

lattice formula formula weight space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z T/K λ/Å ρcalcd/g cm−3 μMo Kα/mm−1 θmax/deg no. of data collected no. of data no. of parameters R1 [I > 2σ(I)] wR2 [I > 2σ(I)] R1 [all data] wR2 [all data] GOF Rint

CpMo(PMe3)2(CO)H monoclinic C12H24MoOP2 342.19 P21/c 14.2974(13) 9.5684(8) 12.3878(11) 90 111.8330(10) 90 1573.1(2) 4 130(2) 0.71073 1.445 1.018 32.377 26205 5482 154 0.0273 0.0619 0.0363 0.0660 1.072 0.0325 [Cp*Mo(μ-O)(μ-O2CH)]2

monoclinic C12H12Mo2O6 444.10 C2/c 19.104(10) 6.102(3) 12.999(7) 90 122.780(6) 90 1274.1(11) 4 130(2) 0.71073 2.315 1.991 32.327 21420

orthorhombic C22H32Mo2O6 584.35 Pnma 14.055(9) 20.208(13) 7.933(5) 90 90 90 2253(3) 4 130(2) 0.71073 1.723 1.148 26.372 13020

2243 91 0.0347 0.0827 0.0435 0.0860 1.079 0.0697

2376 183 0.0580 0.1267 0.0855 0.1372 1.206 0.0910

the reaction was monitored by 1H NMR spectroscopy. After a period of 5 days, all of the formic acid was consumed, and hydrogen, methanol, methyl formate, Cp*Mo(CO)3H, [Cp*Mo(CO)3]2,90 and [Cp*Mo(μ-O)(μ-O2CH)]2 were observed by 1H NMR spectroscopy. Another aliquot of formic acid (10 μL, 0.27 mmol) was added, and the mixture was heated to 100 °C until the formic acid was consumed. H

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This procedure was repeated over a period of 21 days with a total of three aliquots of formic acid, until the predominant species in the bluegreen solution was [Cp*Mo(μ-O)(μ-O2CH)]2. The mixture was lyophilized to produce [Cp*Mo(μ-O)(μ-O2CH)]2 as a blue-green powder (2 mg, 22% yield). Blue-orange dichroic crystals suitable for Xray diffraction were obtained via slow evaporation of a solution in benzene. Anal. Calcd for [Cp*MoO(O2CH)]2: C, 45.22%; H, 5.52%. Found: C, 45.03%; H, 5.24%. 1H NMR (C6D6): 1.82 [s, 30H, [(C5Me5)Mo(μ-O)(μ-O2CH)]2], 7.77 [s, 2H, [(C5Me5)Mo(μ-O)(μO2CH)]2]. 13C{1H} NMR (C6D6): 9.88 [s, 10C, [(C5Me5)Mo(μO)(μ-O2CH)]2], 114.44 [s, 10C, [(C5Me5)Mo(μ-O)(μ-O2CH)]2], 164.13 [s, 2C, [(C5Me5)Mo(μ-O)(μ-O2CH)]2]. IR (cm−1): 2914 (w), 2852 (w), 2031 (w), 1949 (w), 1724 (w), 1660 (w), 1556 (s), 1483 (w), 1447 (w), 1375 (m), 1345 (s), 1261 (m), 1096 (m), 1023 (m), 801 (m), 777 (m), 709 (w), 665 (m), 623 (w), 591 (w), 470 (w), 442 (m), 424 (m). 13 C Isotope Exchange between [CpMo(μ-O)(μ-O2CH)]2 and H13CO2H. [CpMo(μ-O)(μ-O2CH)]2 (3 mg, 0.007 mmol) was added to a solution of H13CO2H (20 μL, 0.5 mmol) in C6D6 (ca. 0.7 mL), and the mixture was heated to 100 °C. The reaction was monitored by 1 H and 13C{1H} NMR spectroscopy, which demonstrated the formation of [CpMo(μ-O)(μ-O2 13 CH)]2 with no observable liberation of 13CO2.

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Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the U.S. Department of Energy, Office of Basic Energy Sciences (DE-FG02-93ER14339) for support of this research. M.C.N. acknowledges the National Science Foundation for a Graduate Research Fellowship under Grant DGE 1144155. Professor Clark Landis is gratefully acknowledged for providing helpful advice. I

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Inorganic Chemistry

(44) Bottomley, F.; Chen, J. Organometallic oxides - Oxidation of [(η5-C5Me5)Mo(CO)2]2 with O2 to form syn-[(η5-C5Me5)MoCl]2(μCl)2(μ-O), syn-[(η5-C5 Me5)MoCl] 2(μ-Cl)(μ-CO3H)(μ-O), and [C5Me5O][(η5-C5Me5)Mo6O18]. Organometallics 1992, 11, 3404− 3411. (45) Shin, J. H.; Parkin, G. Phosphorus-carbon bond activation of PMe3 at a dimolybdenum center: synthesis and structure of [Cp*Mo(μ-O2CMe)]2(μ-PMe2)(μ-Me). Chem. Commun. 1998, 1273−1274. (46) Kamenar, B.; Penavić, M.; Marković, B. Structure of triammonium μ-formato-(O,O′)-di-μ-oxo-bis[diformato(oxo)molybdate(V)]. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1987, 43, 2275−2277. (47) Modec, B.; Dolenc, D. Molybdenum(V) complexes with formate: Geometric isomerism of the [Mo2O4Cl2(py)2(HCOO)]− ion. J. Mol. Struct. 2013, 1051, 354−360. (48) Takehara, M.; Hashimoto, M. Potassium, rubidium and ammonium salts of μ-(formato-κ 2 O:O′)-μ-oxido-bis[oxidobis(peroxido-κ2O,O′)molybdate(VI). Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2013, 69, 37−40. (49) Adams, R. D.; Collins, D. M.; Cotton, F. A. Inorg. Chem. 1974, 13, 1086−1090. (50) Zhang, X.; Meng, D.; Li, X.; Meng, L.; Sun, Z. Nature of the MM bonding (M = Cr, Mo, and W) in [CpM(CO)3]2: Covalent single bond or noncovalent interaction? J. Organomet. Chem. 2014, 769, 106−111. (51) Clegg, W.; Compton, N. A.; Errington, R. J.; Norman, N. C. Structure of hexacarbonylbis(pentamethylcyclopentadienyl)dimolybdenum(Mo-Mo). Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1988, 44, 568−570. (52) Cotton, F. A.; Daniels, L. M.; Hillard, E. A.; Murillo, C. A. The lengths of molybdenum to molybdenum quadruple bonds: Correlations, explanations, and corrections. Inorg. Chem. 2002, 41, 2466− 2470. (53) Cotton, F. A.; Norman, J. G.; Stults, B. R.; Webb, T. R. The preparation and crystal structure of dimolybdenum tetraformate; Photoelectron spectra of this and several other dimolybdenum tetracarboxylates. J. Coord. Chem. 1976, 5, 217−223. (54) Shorter Mo−Mo bond lengths have been reported for quintuply bonded [Mo{μ-κ2-PhC(N-2,6-Pri2C6H3)2}2]2 [2.0157(4) Å] and [Mo{μ-κ2-HC(N-2,6-Pri2C6H3)2}2]2 [2.0187(9) Å]. See: (a) Tsai, Y.-C.; Chen, H.-Z.; Chang, C.-C.; Yu, J.-S. K.; Lee, G.-H.; Wang, Y.; Kuo, T.-S. Journey from Mo-Mo quadruple bonds to quintuple bonds. J. Am. Chem. Soc. 2009, 131, 12534−12535. A shorter bond length of 2.0123(12) Å has also been reported for [Mo{μ-κ2-HC(N-2,6Pri2C6H3)2}2]2 in a cocrystallized sample. See: (b) Lu, D.-Y.; Chen, P. P.-Y.; Kuo, T.-S.; Tsai, Y.-C. The Mo-Mo quintuple bond as a ligand to stabilize transition-metal complexes. Angew. Chem., Int. Ed. 2015, 54, 9106−9110. (55) Bennett, M. J.; Brencic, J. V.; Cotton, F. A. The preparation and structural characterization of trirubidium octachlorodimolybdenum. A binuclear structure with strong metal-metal bonding. Inorg. Chem. 1969, 8, 1060−1065. (56) Cotton, F. A. Discovering and understanding multiple metal-tometal bonds. Acc. Chem. Res. 1978, 11, 225−232. (57) Cotton, F. A.; Walton, R. A. Multiple Bonds between Metal Atoms, 2nd ed.; Oxford University Press: Oxford, U.K., 1993. (58) Indeed, the metal−metal separation in dinuclear complexes with bridging ligands and no metal−metal bond may be shorter than that in unbridged complexes with a metal−metal bond. For example, see: Vahrenkamp, H. What do we know about the metal-metal bond? Angew. Chem., Int. Ed. Engl. 1978, 17, 379−392. (59) (a) Baik, M.-H.; Friesner, R. A.; Parkin, G. Theoretical investigation of the metal−metal interaction in dimolybdenum complexes with bridging hydride and methyl ligands. Polyhedron 2004, 23, 2879−2900. (b) Parkin, G. Metal−metal bonding in bridging hydride and alkyl compounds. Struct. Bonding (Berlin, Ger.) 2010, 136, 113−146.

komplexe des Chroms, Molybdans und Wolframs: C5H5-, C5Me5- und ein isoelektronischer 6e-sauerstof-tripodligand im vergleich. J. Organomet. Chem. 1987, 331, 317−327. (28) For related NMR spectroscopic studies on CpMoL(CO)2X, see: Faller, J. W.; Anderson, A. S. Organometallic conformational equilibria. XI. Cis-trans isomerism and stereochemical nonrigidity in cyclopentadienylmolybdenum complexes. J. Am. Chem. Soc. 1970, 92, 5852−5860. (29) It is interesting to note that isomerization of CpRMoL(CO)2I derivatives has also been observed in the solid state. See: Adeyemi, A. G.; Eke, U. B.; Cheng, L.; Cook, L. M.; Billing, D. G.; Mamba, B. B.; Levendis, D. C.; Coville, N. J. Solid-state isomerisation reactions of (η5-C5H4R)M(CO)2(PR3′)I (M = W, Mo; R = tBu, Me; R′ = Ph, OiPr3). J. Organomet. Chem. 2004, 689, 2207−2215. (30) Antunes, M. A.; Namorado, S.; de Azevedo, C. G.; Lemos, M. A.; Duarte, M. T.; Ascenso, J. R.; Martins, A. M. Pentabenzylcyclopentadienyl molybdenum and tungsten hydrides: Syntheses, structures and electrochemistry of [MHCpBz(CO)2(L)] (L = CO, PMe3, PPh3). J. Organomet. Chem. 2010, 695, 1328−1336. (31) Brammer, L.; Zhao, D.; Bullock, R. M.; McMullan, R. K. X-ray and neutron diffraction studies of tricarbonyl(ηpentamethylcyclopentadienyl)hydridomolybdenum at 163 K. Inorg. Chem. 1993, 32, 4819−4824. (32) Namorado, S.; Cui, J.; de Azevedo, C. G.; Lemos, M. A.; Duarte, M. T.; Ascenso, J. R.; Dias, A. R.; Martins, A. M. (Pentabenzylcyclopentadienyl)molybdenum complexes: Synthesis, structures and redox properties. Eur. J. Inorg. Chem. 2007, 2007, 1103−1113. (33) van der Eide, E. F.; Yang, P.; Bullock, R. M. Isolation of two agostic isomers of an organometallic cation: different structures and colors. Angew. Chem., Int. Ed. 2013, 52, 10190−10194. (34) Nakazawa, H.; Ohba, M.; Itazaki, M. Synthesis and reactivity of a phosphite−boryl complex of molybdenum: Formation of (C5Me5)Mo(CO)3(BH2phosphite) and its Mo−B, B−P, and B−H bond reactions. Organometallics 2006, 25, 2903−2905. (35) A possible source of the oxo ligands is water that is present in formic acid or generated via disproportionation of formic acid. (36) While [CpMo(CO)3]2 can re-enter the catalytic cycle, [CpMo(μ-O)(μ-O2CH)]2 does not serve as a catalyst for decarboxylation of formic acid, although treatment with H13CO2H results in exchange of the formate ligand. (37) Kubas, G. J.; Kiss, G.; Hoff, C. D. Solution calorimetric, equilibrium, and synthetic studies of oxidative addition/reductive elimination of C5R5H (R = H, Me, Indenyl) to/from the complexes M(CO)3(RCN)3/(η5-C5R5)M(CO)3H (M = Cr, Mo, W). Organometallics 1991, 10, 2870−2876. (38) Landrum, J. T.; Hoff, C. D. The heats of hydrogenation of the metal-metal bonded complexes [M(CO)3C5H5]2 (M = Cr, Mo, W). J. Organomet. Chem. 1985, 282, 215−224. (39) Bunker, M. J.; Green, M. L. H. Mono-η-cyclopentadienylmolybdenum chemistry - Some oxo-derivatives, oxohalogeno-derivatives, halogeno-derivatives, thio-derivatives, η-disulphido-derivatives, and thiohalogeno-derivatives. J. Chem. Soc., Dalton Trans. 1981, 847−851. (40) Demirhan, F.; Richard, P.; Poli, R. High oxidation state aqueous organometallics: synthesis and structure of a dinuclear oxo(pentamethylcyclopentadienyl)acetato complex of molybdenum(IV), [Cp*Mo(μ-O)(μ-O2CCH3)]2. Inorg. Chim. Acta 2003, 347, 61−66. (41) Searches of the Cambridge Structural Database were performed with version 5.37. See: Groom, C. R.; Bruno, I. J.; Lightfoot, M. P.; Ward, S. C. The Cambridge Structural Database. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2016, 72, 171−179. (42) Le Hénanf, M.; Le Roy, C.; Muir, K. W.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J. Electrochemical studies of complexes with oxo- or hydroxo-bridged {Mo2(μ-SMe)3}+ centers: Cleavage of the oxygen bridge and generation of substrate-binding sites. Eur. J. Inorg. Chem. 2004, 2004, 1687−1700. (43) Alvarez, M. A.; García, M. E.; Menéndez, S.; Ruiz, M. A. Reactions of the carbyne-bridged radical complex [Mo2(η5-C5H5)2(μCPh)(μ-PCy2)(μ-CO)]+ with bidentate ligands having E-H bonds (E = O, S, N). Organometallics 2014, 33, 1181−1189. K

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Inorganic Chemistry (60) It is worth noting that, as a consequence of the fact that the Mo−Cpcent vectors are not in alignment with the Mo−Mo vector (x axis), the side-on overlap of two dyz orbitals to give δ and δ* orbitals causes the derived orbitals to possess some π character. Likewise, overlap of two dxz orbitals to give π and π* orbitals causes the derived orbitals to possess some σ character. Such orbitals have been described as “slipped” δ/δ* and π/π* orbitals, but here they are simply called idealized δ/δ* and π/π* orbitals for brevity. See ref 59a and: Wilson, Z. S.; Stanley, G. G.; Vicic, D. A. To bend or not to bend: Electronic structural analysis of linear versus bent M-H-M interactions in dinickel bis(dialkylphosphino)methane complexes. Inorg. Chem. 2010, 49, 5385−5392. (61) Coulson, C. A. The electronic structure of some polyenes and aromatic molecules. VII. Bonds of fractional order by the molecular orbital method. Proc. R. Soc. London, Ser. A 1939, 169, 413−428. (62) Mayer, I. Bond order and valence indices: A personal account. J. Comput. Chem. 2007, 28, 204−221. (63) For another example of a dimolybdenum(IV) compound with a formal bond order of zero, see ref 44. (64) Shaik, S.; Hoffmann, R.; Fisel, C. R.; Summerville, R. H. Bridged and unbridged M2L10 complexes. J. Am. Chem. Soc. 1980, 102, 4555− 4572. (65) Cotton, F. A. The structures of metal-metal-bonded edgesharing bioctahedral complexes. Polyhedron 1987, 6, 667−677. (66) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 6th ed.; Wiley-Interscience: New York, 1999. (67) Zurek, J. M.; Paterson, M. J. Theoretical study of the pseudoJahn-Teller effect in the edge-sharing bioctahedral complex Mo2(DXyIF)2(O2CCH3)2(μ2-O)2. Inorg. Chem. 2009, 48, 10652− 10657. (68) Anderson, L. B.; Cotton, F. A.; DeMarco, D.; Fang, A.; Ilsley, W. H.; Kolthammer, B. W. S.; Walton, R. A. Experimental and theoretical evidence for double-bonds between metal atoms - Dinuclear alkoxobridged ditungsten(IV,IV) complexes, W2Cl4(OR)4(ROH)2. J. Am. Chem. Soc. 1981, 103, 5078−5086. (69) Chiarella, G. M.; Cotton, F. A.; Murillo, C. A.; Zhao, Q. A strong metal-to-metal interaction in an edge-sharing bioctahedral compound that leads to a very short tungsten-tungsten double bond. Inorg. Chem. 2014, 53, 2288−2295. (70) Poli, R.; Torralba, R. C. Further considerations on the structure and bonding in edge-sharing bioctahedral complexes. Inorg. Chim. Acta 1993, 212, 123−134. (71) (a) Abugideiri, F.; Fettinger, J. C.; Poli, R. Metal-metal bonding in pentamethylcyclopentadienylmolybdenum(IV) dinuclear compounds - Chloride abstraction from nonbonded Cp*2Mo2Cl6 to afford bonded [Cp*2Mo2Cl5]. Inorg. Chim. Acta 1995, 229, 445−454. (b) Chakravarty, A. R.; Cotton, F. A.; Diebold, M. P.; Lewis, D. B.; Roth, W. J. A series of edge-sharing bioctahedral, M-M bonded molecules - Nonmonotonic bond length variation and its interpretation. J. Am. Chem. Soc. 1986, 108, 971−976. (c) Cotton, F. A.; Diebold, M. P.; O’Connor, C. J.; Powell, G. L. Edge-sharing bioctahedral dimolybdenum(III) molecules with μ-RS groups. Direct experimental-evidence for spin-state equilibria. J. Am. Chem. Soc. 1985, 107, 7438−7445. (d) Poli, R.; Mui, H. D. Mononuclear octahedral and dinuclear edge-sharing and face-sharing bioctahedral compounds of molybdenum(III). Electronic control on the extent of metal-metal interaction in the dinuclear systems. An equilibrium, structural, and paramagnetic NMR study. Inorg. Chem. 1991, 30, 65−77. (72) (a) Green, J. C.; Green, M. L. H.; Parkin, G. The occurrence and representation of three-centre two-electron bonds in covalent inorganic compounds. Chem. Commun. 2012, 48, 11481−11503. (b) Green, M. L. H.; Parkin, G. Application of the Covalent Bond Classification Method for the teaching of inorganic chemistry. J. Chem. Educ. 2014, 91, 807−816. (c) Green, M. L. H.; Parkin, G. The Covalent Bond Classification Method and its application to compounds that feature three-center two-electron bonds. Struct. Bonding (Berlin, Ger.) 2016, 171, 79−140.

(73) We note that the structure−bonding representation of [CpMo(μ-O)(μ-O2CH)]2 shown in Figure 10 is only one possibility; others that include symmetric μ-L donation from the bridging oxo atoms can also be included. See refs 72a and 72c. (74) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Slaton, J. G. A pseudo-Jahn-Teller distortion in an Mo2(μ2-O)2 ring having the shortest MoIV-MoIV double bond. J. Am. Chem. Soc. 2002, 124, 2878− 2879. (75) For a manganese compound with two bridging oxo ligands and two bridging acetate ligands that has a minimal Mn···Mn interaction, see: Petrie, S.; Mukhopadhyay, S.; Armstrong, W. H.; Stranger, R. Theoretical analysis of the [Mn2(μ-oxo)2(μ-carboxylato)2]+ core. Phys. Chem. Chem. Phys. 2004, 6, 4871−4877. (76) Wiberg, K. B. Application of Pople-Santry-Segal CNDO method to cyclopropylcarbinyl and cyclobutyl cation and to bicyclobutane. Tetrahedron 1968, 24, 1083−1096. (77) (a) Mayer, I. Charge, bond order and valence in the ab initio SCF theory. Chem. Phys. Lett. 1983, 97, 270−274. (b) Mayer, I. Charge, bond order and valence in the ab initio SCF theory. Chem. Phys. Lett. 1985, 117, 396. (c) Mayer, I. Charge, bond order and valence in the ab initio SCF theory. Int. J. Quantum Chem. 1986, 29, 73−84. (78) Bridgeman, A. J.; Cavigliasso, G.; Ireland, L. R.; Rothery, J. The Mayer bond order as a tool in inorganic chemistry. J. Chem. Soc., Dalton Trans. 2001, 2095−2108. (79) Glendening, E. D.; Weinhold, F. Natural Resonance Theory: II. Natural bond order and valency. J. Comput. Chem. 1998, 19, 610−627. (80) (a) Weinhold, F. Natural bond orbital analysis: A critical overview of relationships to alternative bonding perspectives. J. Comput. Chem. 2012, 33, 2363−2379. (b) Glendening, E. D.; Landis, C. R.; Weinhold, F. Natural bond orbital methods. WIREs Comput. Mol. Sci. 2012, 2, 1−42. (81) For some articles that discuss the usefulness of various methods to determine bond orders, see: (a) Gonthier, J. F.; Steinmann, S. N.; Wodrich, M. D.; Corminboeuf, C. Quantification of ‘fuzzy’ chemical concepts: a computational perspective. Chem. Soc. Rev. 2012, 41, 4671−4687. (b) Wagner, F. R.; Noor, A.; Kempe, R. Ultrashort metalmetal distances and extreme bond orders. Nat. Chem. 2009, 1, 529− 536. (c) Olah, J.; Blockhuys, F.; Veszprémi, T.; Van Alsenoy, C. On the usefulness of bond orders and overlap populations to chalcogennitrogen systems. Eur. J. Inorg. Chem. 2006, 2006, 69−77. (d) Lu, T.; Chen, F. Bond order analysis based on the Laplacian of electron density in fuzzy overlap space. J. Phys. Chem. A 2013, 117, 3100−3108. (82) (a) Glendening, E. D.; Weinhold, F. Natural Resonance Theory: I. General formalism. J. Comput. Chem. 1998, 19, 593−609. (b) Glendening, E. D.; Weinhold, F. NBO 6.0 Program Manual; Board of Regents of the University of Wisconsin System on behalf of the Theoretical Chemistry Institute: Madison, WI, 1996−2013. (83) (a) McNally, J. P.; Leong, V. S.; Cooper, N. J. Cannula techniques for the manipulation of air-sensitive materials. In Experimental Organometallic Chemistry; Wayda, A. L., Darensbourg, M. Y., Eds.; American Chemical Society: Washington, DC, 1987; Chapter 2, pp 6−23. (b) Burger, B. J.; Bercaw, J. E. Vacuum line techniques for handling air-sensitive organometallic compounds. In Experimental Organometallic Chemistry; Wayda, A. L., Darensbourg, M. Y., Eds.; American Chemical Society: Washington, DC, 1987; Chapter 4, pp 79−98. (c) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-Sensitive Compounds, 2nd ed.; Wiley-Interscience: New York, 1986. (84) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. NMR chemical shifts of trace impurities: Common laboratory solvents, organics, and gases in deuterated solvents relevant to the organometallic chemist. Organometallics 2010, 29, 2176−2179. (85) (a) Sheldrick, G. M. SHELXTL: An Integrated System for Solving, Refining, and Displaying Crystal Structures from Diffraction Data; University of Göttingen: Göttingen, Germany, 1981. (b) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (c) Sheldrick, G. M. SHELXT − L

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DOI: 10.1021/acs.inorgchem.6b02606 Inorg. Chem. XXXX, XXX, XXX−XXX