The Crystal Structures of Trimethylplatinum Chloride and

---

July, 1947 CRYSTAL STRUCTURES OF TRXMRTBYL~LATINUM C h m m E AND TETRAMETHYLPLATINUM 1681

for from the center of the molecule along the body with the calculations, to Mr. Kent E prspating the Fourier projBction, and to D r. &gcnd. The same effect would not be. teptized with the carbon atoms because the two hydrogen Lindsay Helmholz for help with the low-kmbpnds conaecting with each carbon atom make an perature photographs. angle of 150’8’ with one another and the two tenS-w sions cancel for the most part. A comparison of X-ray data obtained a t low The existence of the hydrogen bonds between adjacent molecules, however, while expected to temperature with those obtained a t roam temreduce the pssibility of isotropic t h d vibra- perature indicates the existence of rotational thertion, does not greatly interfere with rotational mal vibrations in the hexamethylenetetramine vibrations of the molecule. Each nitrogen is a t crystal. Structure amplitudes calculated on the the top of a triangular pyramid whose base has a basis of assumed rotational vibrations are iu carbon atom of each vertex. The hydrogen bonds closer agreement with the observed amplitudes for form the three near-vertical edges of this figure. room temperature than are those calculated withDisplacement of the nitrogen 0.26 A. ig any di- out the use of a rotational temperature factor. rection parallel to the base may vary a hydrogen A slightly altered structure which provides a Mtbond length by not more than 0.11 A., and leaves ter fit than any other structure considered is derived by the method of least squares. The bond the average virtually unchanged. angles and bond distances are C-N = 1.45 f Acknowledgment.-I wish to express my grati- 0.01 A.; C-N-C = 107’; N-C-N = 113’30’. tude to Professor Linus Pauling for suggesting The carbon-nitrogen bond distance is about 0.03 this work to me and for the many helpful sug- A. less in the crystal than in the gas. gestions which he made during its course, and to CALIFORNU RBCELVBD(MARCH1,1847 Professor Verner Schomakpr for valuable discus- PASADENA, (9) Original manuscript received July 29, 1946. sions. I am indebted to my wife for assistance [CONTRIBUTION FXOY

THE

GAITS AND CRBLLINL A B O R A T OOF ~ ~CHBXISI’XY, ~S CALIFORNIA INSTITUTE OF l ” m y ,

No. 10871

The Crystal Structures of Trimethylplatinum Chloride and Tetramethylpiatixmm BY R. E. RUNDLE’AND J. H.STURDNANT All reported structures of quadrivalent platinum compounds have involved octahedral coordination about platinum. Presumably, however, quadrivalent phtinum is czapable of forming d’sp tebonds, and the platinum alkyls, tetramethylplatinum, hexamethyldiplatinum, trimethylplatinum chloride, etc., were expected to have this configuration.* Cox and Webstera determined the correct unit cell with a = 10.52 A. for trimethylplatinum c h l e d e from powder diagrams, but no real attempt was made to examine the structure. Since no structures involving d2@ tetrahedral bondhg have been verified, it seemed important to investigate the structures of some of the above-named compounds. Experimental Prepurtion of the Compounds.-Trimethylplatinum chloride was prepared by the action of platinum tetrachloride on methyl Grignard reagent as described by Pope and Peachey,‘ except that the platinum tetrachloride was made according to the directions of Kharasch and Ash(1) Resent addnar: Department of Chemistry. Iowa State College, Am-, Iowa. (2) Linw Pading, “The Naturc of the C M c a l Bond,” 2nd ed., Cornell University Pres, Ith.a, New York, 1944, p. 102. (8) E. 0. Cox and K.C. Webata, 2 . Krisl., W, 661 (leSa). (4) W.J. Pope and S. J. Pachey, 1. C h . Soc., $6, 571 (1909).

ford,l and was added to the Grignard reagent as the dry powder as recommended to us by Professor Gilman. The material had the properties described by Pope and Peachey‘ for trimethylplatinum chloride, and X-ray powder linesproduced by it agreed with the measurements of Cox and WebSter.’

Tetramethylplatinum6was furnished us by Professor Gilman. Unlike other compounds of the type M a it is a crystalline material. It is likewise less soluble in organic solvents than are other compounds of this formula. Crystal System and Unit Ceh.-Both trimethylplatinum chloride and tetramethylplatinum crystallize from benzene in anisotropic plates which quickly lose solvent of crystallization to form isotropic powders. From aliphatic hydrocarbons and the ethers they crystdim 8s rhombic dodecahedra of the cubic system. This habit was confirmed by the optical goniometry of several crystals of each compound. When heated the crystals decompose without melting. By the Laue method two, three, and four-fold axes were found for both crystals in positions corresponding to the goniometry, and Oh-m3m was (5) M.5. Kharasch and T.A. Ashford, Tma ]om&, I , 1 m (1938). (6) Henry Gilman and M. LichtenrrPlter, THIa Jouunt, W, (1938).

R. E.RUNDLEAND J. H. STURDWANT

1582

e!stat&hecl as the h u e symmetry. With the iadiation from a copper target filtered through nickel and a camera of 5cm. radius intense exposures (800 to loo0 millimpere hours at 38 kv. peak) were made with the crystals rotating about two and three-fold axes. These could be indexed on the basis of bodycentered cubic units, with a = 10.55 A. for trimethylplatinum chloride, and a = 10.145 A. for tetramethylplatinum. Powder diagrams also were indexed by use of these units and in the case of tetramethylplatinum the unit w a s tested further by the indexing of Laue diagrams. The density of tetramethylplatinum, as determined by the flotation of crystals in solutions of methylene iodide and methyl iodide, is 3.16 g./cc. The density calculated for eight MeiPt molecules per unit cell is 3.23 g./cc. The density of trimethylplatinum chloride has been founds to be 3.1 g./cc. ; hence it contains eight MeJ'tCl molecules per unit cell. Space Group Discussion.-Though the X-ray data indicate the Law symmetry oh and a bodycentered lattice, it is questionable whether this evidence can be considered binding for the methyl groups, whose scattering power is relatively low. We shall, however, accept it for the arrangement of the platinum atoms in both crystals, and for the chlorine atoms in trimethylplatinum chloride. Possible space groups for these atoms are then' Td'-I&m, -143, 0-1413, and Ohg-Im3m. The positions7 for eight platinum atoms, when the origin is chosen suitably in each case, are the following : (OOO;

'/3

'/2

I/:)

+

(a) OOO; 0 I/s '/s;

(8) '/n

'/a '/n; ( 7 ) n/' '/a '/a; (6) u u u; u

'/a '/a

I/¶

0 I/¶; '/n; '/a

'/a '/a '/a;

n/'

'/a '/a; '/a;

'/a

'/a

'/n

'/n

'/n. '/n.

ii ii; ii u ii; ii Ti u.

Atoms occupying the set of positions (a)make no contribution to the intensity of maxima with hl, b,or hs odd, Many reflections of the form (2n 1, 2n 1, 2n) are so intense on rotation diagrams from both compounds that their reflections must necessarily involve a large platinum contribution. The positions (a) are therefore eliminated for platinum. The sets (8) and (y) are enantiomorphs, and intensities calculated for an array of platinum atoms alone in either of these sets are in rough agreement with those observed for both compounds. If the platinum atoms are in (6) the parameter u is limited to the neighborhood of * l/8 or *3/8; these values bf u produce congruent or enantiomorphic motifs; their intensities are identical with those from (8) or (7). Then ( P ) , (y), and ( 6 ) offer possible platinum positions. If the platinum atoms were in (8) or (r),the crystals of trimethylplatinum chloride and tetramethylplatinum could be optically active; actually an examination failed to reveal optical ac-

+

The negative result of this sensitive test sugglests strongly that the positions (8) and ( 7 ) &odd be considered no further. Fortunately, for both crystsls it is possible to eliminatie &e p0,sitions (8) and ( 7 )on other evidence as well. Platinum atoms in positions (8)or ( 7 ) contribute nothing to reflections of the forms (4n,4n, 4n 2) and (4n 2,4n 2,4n). These reflections appear to be absent for trimethylplatinum chloride, but for large values of the Bragg angle several such retlections were observed from tetramethylplatinum. The intensities of these. refIections are too great to be explained on the basis of the scattering of the carbon atoms alone; hence the positions (8) and ( 7 ) are eliminated for the platinum atoms in tetramethylplatinurn. For this compound the platinum atoms must be in (6) with u near,but not e ual to, or 3/8. The positions (8) and 77)can be eliminated for the platinum atoms in trimethylplatinum chloride only by considering the chlorine positions along with the platinum positions. The positions available for the eight chlorine atoms are again (a), (8), (y), and (6). With the chlorine atoms in (a) and the platinum atoms in (8), (r),or (a), it is easily shown that reflections of the form (4n 2, 4n 2, 4n 2) should be stronger than reflections (4n, 4n, 4n) with n odd. In every case where such reflections occur in the same vicinity the opposite is true. The differenceis too great to be accounted for by carbon contributions, and the set (a)is thus eliminated for the chlorine atoms. With the platinum atoms in (8) the only remaining set of positions for the chlorine atoms consistent with the cubic symmetry is (y). If we consider reflections of the type (2n 1, 2n' -j-1, 2n') we find that for the above structure the structure factors fall into two groups. For n, n', nw even, for n, nu even, n' odd, and for n, n' odd, n" even, F = *4i(fpt - fcl). For n, n' even, n" odd, for n, n" odd, n' even, and for n, n', nwodd, F = f4(fpt fcl). Examination of a large number of reflections of the above type revealed that reflections of the first group were more intense than those of the second whenever comparisons were possible. The structure under consideration is, therefore, eliminated. The enantiomorphic structure with the chlorine atoms in (8) and the platinum atoms in (y) need not be considered. The positions for both platinum atoms and chlorine atoms must be (6). As will be seen, these positions provide a satisfactory structure. Of the space groups previously listed, only Td3-11?3m is permitted. It is true, however, that the space groups T d ' - P & z and Td4Pi3n furnish the positions chosen for the platinum and chlorine atoms. If the methyl groups are not arranged on a body-centered lattice one of these space groups must be chosen for the structure. For the space group Td'-P&, the point symmetry of the platinum and chlorine atoms is Cs0tivity,

+

+

+

+

+

+

+

0. '/a

Vol. 69

+

(7) "International Tables for the Determination of Crystal StrueCures," Gebruder Borntraeger, Berlin, 1935, Vol. I.

+

JL@,1947 CRYSTAL STRUCTURESOF TRIMETHYLPLATIN'WM CHLORIDE AND TETRAMETHYLPLATINUM 1563 3m, as it is for Td'-I&m, but each set of positions is divided into two non-equivaknt sets of four positions. No arrangement of the methyls seems to call for non-equivalence of the eight platinum or chlorine atoms. In Td4the eight positions in each set are equivalent but have the point symmetry C3 - 3. Again it seems unnecessary to consider for the methyls distorted configurations which would lower the point symmetry of the platinum and chlorine atoms. For these reasons, the space group Td3-I&n was chosen for both crystals; that it provides satisfactory structures is added justification for this choice. Determination of the Parameters for Trimethytplatinum Chloride.-With the platinum atoms in (6) with upt = both rough intensity considerations and steric factors limit the chlorine parameter to the neighborhood of For a more accurate determination of the platinum and chlorine parameters, it was ncpssary to obtain good intensity data, corrected accurately for the high absorption of platinum. For this purpose a small crystal was attached by shellac to a glass fiber, with the fiber axis parallel to a two-fold axis of the crystal. The crystal was well centered on the fiber and the shellac was baked until firm. The crystal was then ground to a cylinder on a milling machine using a rotating ground glass plate as the grinding surface. Under the microscope the crystal appeared to be a smooth cylinder 0.14 mm. long, 0.1 mm. in diameter. This crystal was used in preparing intense rotation diagrams with CuKa radiation filtered through nickel foil 70 p thick; a t 38 kv. p. exposures ran to 975 milliampere hours. The interference maxima produced very symmetrical, clean spots on the photographic film. Relative intensities were visually estimated by the multiple film technique.8 The absorption correction was made according to Claassen9; the linear absorption coeffiaent equals 471 for trimethylplatinum chloride and X = 1.54 A. The intensity data were analyzed in the steps outlined in the following paragraphs. The reflection (929) is easily visible, but (2.12.2) was not observed. This requires upt < 0.385 regardless of the chlorine and carbon parameters. That (929) is but a fourth as intense as (6.10.6) requires upt > 0.370, again regardless of the chlorine and carbon contributions to these reflections. If, in accordance with our earlier observation, we restrict UCI to 0.125 * 0.025, and then add the most unfavorable possible combination of contributions from chlorine and carbon atoms to the variable contribution from platinum atoms, we obtain the relative intensities for (929), (2.12.2), and (6.10.6) which are plotted in Fig. 1 as functions of upt. The most unfavorable conditions m e r widely, as shown by the break in the (929) curve. Any reasonable compromise leads to upt = 0.375 * 0.002. (8)

J. J. de Lange, J. Monteath Robertson and I. Woodward.

Ptoc. Roy. SOC.(London), AlTl, 404 (1939). (9) CY. (7), Vol. 11, pp. 583-585.

I

I

I

-

c El

40-

0.366

0.370

0.375

0.380

0.385

UP&.

Fig. 1.-Determination of the platinum parameter in trimethylplatinum chloride.

The reflections (4n,4n, 4n) are more intense than reflections (4s 2, 4n 2,4n 2). The contributions of the platinum atoms to the structure factors of these reflections are very closely equal in magnitude even for small variations in the platinum parameter; hence these reflections are most useful in determining the chlorine parameter. Thus the observation (4.12.i) > (6.10.8) requires that 0.100 < UCI < 0.150 regardless of the carbon contribution. Further refinement of the chlorine parameter demands that some account be taken of the positions of the carbon atoms. These cannot be determined from X-ray evidence because of the low scattering power of carbon relative to platinum; but from the structure as determined thus far the rough positions of the carbon atoms become apparent. Thus from the positions of the platinum and chlorine atoms it is obvious that trimethylplatinum chloride i s a tetramer (Figs.2 and 3), with each platinum atom bonded to three chlorines, each chlorine to three platinums. Four chlorine atoms together with four platinum atoms form a rough cube, the platinum atoms forming one tetrahedron, the chlorine atoms its negative, Chemically, we should expect three methyls to be bonded to each platinum atom. This arrangement is achieved by filling out the octahedron about platinum, giving platinum the same type of coordination that it shows in all other quadrivalent compounds. The carbon atoms can then be placed in the positions' 24 (g) of the space p u p Tda-Ii3m. If the carbon-platinum distance i s assumed to equal the s u m of the covalent radii (2.08 K . ) , * O then xc 0.375 and zc 0.18. To permit reiinement of the chlorine parameters these carbon parameters need be only exact enough to yield approximate values of the contri-

+

+

(10) L h w Pauling, ref. 2, pp. 164 and 182.

+

R. E. RUNDLEAND J. H.STURDWANT

1564

Vol. 69

butions of the carbon atoms to the various reflections.

0.10

@=pr

0.12

0.13

0.14

0.15

uc1.

- CL

IN PTME~CL

ME IN ME, F i 2.-The

0.11

tetramer which occurs in crystalline trimethylplatinum chloride and tetramethylplatiaum.

Fig. 4.-Determinatipn of the chlorine parameter in trimethylplatinum chloride.

drical crystal. The symbols in the table have the following significance I&,. = I..p.K A (sin 29)/(l

+ cos*28) c--krh’

where I-,,. is the experimentally observed intensity of reAection, K is a convenient constant, A is the absorption correction, and k, in the exponent of the temperature-correction factor, was given the value 0.022; and I:.lod.

= K‘FF*/

where K‘ is a convenient constant, F is the structure amplitude and F* its complex conjugate, and J is the multiplicity factor. The agreement between observed and calculated intensities is seen to be satisfactory. Determination of the Parameters for Tetramethylpl,atinum.-For this compound the platinum parameter alone could be determined from X-ray data. It .was hoped that some direct information could be obtained concerning the carbon positions, but this hope proved vain, in part because of the difficulty in making reliable absorption corrections. Attempts to prepare cyFig. 3.--Tht cubic d t d structure of aystalline tri- lindrical crystals failed due to the instability and m-lplatinum chlaride and tetramethylplatinum; the insolubility of the compound, Since, however, tetramer shown m Fig. 2 is repeated by parallel displacetetramethylplatinum crystallizes in the same form ment at the points of a bodyentered cubic lattice. as trimethylplatinum chloride and has the same absorption coefficient, approximate corrections If we now reconsider the chlorine parameter we could be made by comparing reflections from trifind that (4.12.4) > (&lo.@ requires 0.105 < methylplatinum chloride dodecahedra with reflections from the cylindrical crystal of this comi c ,< ~ 0.145. The observation (5.10.5) > (i.12.i) requires that ZQ S 0.11 (Fig. 4). The best value pound. The observed correction factor for each for the chtarine parameter seems to be ucl = 0.11 plane was used to correct the intensities of corresponding reflections from dodecahedral crystals of t 0.01. To refine the parameter further seems pointless in view of the uncertainty in the carbon tetramethylplatinurn. Except for a few planes noted in Table I1 the corrections were small, and parameterg. Table I hsts experimental and calculated intens- this method appears to be satisfactory. Reflections of the forms (4n,4n,4n 2) and ities for the rdections in the [loll zone. The experimental data were derived as described above (4n 2,4n 2,4n)axe weak, but are definitely from rotation photographs prepared with a cylin- present in many cases. Their presence cannot be

+

+

+

Jdy, 1947 CRYSTAL STRUCTURES OF TRIMETHYLPLATINUM CHLORIDE AND TETRAMETHYLPLATINUM 1565 TABLEI ~ ~ P L A T X N I JCHLORIDE: X INTBNSITIBSOF EQUATORIAL REFLECTI~NS ON [loll ROTATION F’xiommws

hthh

020 i2i

202 222 040 323

245 30-4 343 060 i6i 262

344

505 525 363 080

Exp.

1,.

ioi

}

36-4

626 3a 365

-a 384

707 0.10.0

1.io.i

66G

-

3.10.3

SO8 4.10.3 767

6ss 0.12.0 i.12.i 5.10.5

1

-2 . ia.2

3400 0 2450 0 2125 3730

151 0 196 0 254 546

1800

290

285 0 1300 425

55. 0 366 127

0

0

330 650 1300 170

112 207

Calcd..

180 0 172 0 300 540

180 0 138 0 445 435 91 215 78 0 344 144 2 2 95 330 550 90 100 62 246 125 155 1 0 1

[:;262 0 388

{ l,q 98 300 575 108

600

84

275

150

350

248

320

238

0

0

0

0

295 95 105 0 320 22 0

275 100 116 0 430 29 0

55

80

335

525

115

192

35 80

61 150

0

0

15 0

27 0

250

480

35 45 0

67 87 0

80

150

3.12.2

787 so3

calcd.

{

{Z

;:l

I!

1

287 75 95 1 384 44 2

{: {E { ;;1 51 { 16i 24 1

{E 65 74 8 48

{ X%

A

280 77 80 1 370 40 2 35 64 240 230 60 115 50

165 1 2 30 1 300 145 74 64 8 53 58

40 44 25 45 56 6- . 10.8 125 205 214 180 4.12.4 248 175 230 235 0 Calculated with neglect of the carbon scattering.

im

attributed to scattering by carbon atoms, but these reflections are particularly sensitive to the platinum parameter. They require that upt = 0.375 t 0.006; o t h e r w k their calculated intensities become comparable to those of other reflections, which is contrary to observation. Reflection (707) is, however, very weak, even weaker than (O.lO.O), but would become enormous if upt were less than 0.375. The observed inequality, (1.12.1) > (5.10.5), also requires u p t > -0.375 (Fig. 5). The best agreement is obtained if u~ = 0.380 f 0.002. For a comparison of observed and calculated intensities see Table 11. The significance of ILP.is as defined above for Table I, save that the value of k used for tetramethylplatinum is 0.0083. The carbon atoms of the methyl groups which in tetramethylplatinum replace the chlorine atoms of trimethylplatinum chloride were assigned the parameter UC, = 0.11. The carbon atoms of the remaining methyl groups were put into positions analogous to those occupied in trimethylplatinum chloride, with assigned parameters XC,, = 0.375, 2cII= 0.18.

040.0)

0.365

0.370

0.375

0.380

0.385

apt.

of the platinum parameter in tetramethylplatinum.

Fig. 5.-Detennination

Discussion of the Structures.-Both compounds are tetramers with the configuration shown in Fig. 2. The platinum atoms and the chlorine atoms or methyls which replace them lie on three-fold axes and have the point symmetry Ca-3m. Bonded to each platinum are three carbons; the normal to the plane of the carbons is the three-fold axis on which the platinum lies. The point symmetry of the carbon atoms is Crm. There are two of the tetramers per unit cell in a body-centered arrangement, which, for these molecules, is a very close-packed structure (Fig. 3) Though interatomic distances involving carbon atoms had to be assumed, the following distances are of interest. The platinum4orine distance in trimethylplatinum chloride is 2.48 1$., whereas the sutn of the covdent radii is 2.30 A.; the carbon replacing the chlorine is doubtless a t a smaller

R. E.RUNDLEAND J. H. STURDIVANT

1566

Vol. 69

distance from platinum thqn this, but certainly

TABLE I1

the distance is greater than 2.09 A., the sum of the TETUXBTEYLPLATINUM: INTENSITIES OF EQUATORJAL platinum and carbon radii. The shortest platiREFLECTIONS ON [lor] ROTATION hO?OGRAPHS in trimetiylnum-platinum distance is 3.73 I' bh&

ioi 020 i2i

202 222 040

803

} 323

i4i

242

302 843

Exp.

Ca1cd.a

Ca1cd.b

120 0 220 0 590 315

135 1 210 0 585 380

130 0 220 0 600 385 68 105 105 1 310 105 0 ' 0 110 360 490 68 100 93 215 85 115 0 0 0 324 67 88 0 330 44 0 94 53 259 290 70 80 65 130 0 0 40 0 235 140 52 66

260 190 12 255 150 10 150 380 570 130

i6i 26p

343 305 823 368 080

}

290 140 480

363

BOG 282

1

S2G

5@ 865 G4i?

3a

707 0.10.0

0

18 460 24 49 14 350 30 45 130 560

-

3.10.p

808 4.10.3 767

G&

-0 . 1 2 . 0 1.12.i 5.10.5 2.12.2 3.12.3

155 185 120 35 120 9 345 53 14 9

{ 150 1: 7 305

{

6i

89 350 475 110

{ 1165 : :{ 1 6 { 3102: 40 44 9 285

i

9 16 1;:

{ 2: { 1942 114

{

2: 87 15

{E 55 40 26

A.

0

45 49 78 785 38 $09 Calculated for upt a Calculated for up( = 0.380. 0.375. The intensity of tl& reflection is affected especia y strongly by the dodecahedral habit of the crystal used; therefore the absorption correction applied in the derivation of Z&,, is unreliable.

*

-

platinum chloride, 3.44 25. in tetramethylplatinum. The chlorine-chlorine distance is 3.28 A. The angle platinurn-chlorine-platinum is 99'. Methyl-methyl distances between adjacent tetramers are slightly more than 4 A. for the parameters chosen. The unit cell and density data alone are suffiaent to indicate that the expected tetrahedral bonding to platinum does not exist in these compounds. The crystals are far too dense to allow the packing of the expected molecules. The fact that tetramethylplatinum is a crystalline compound rather than a low-boiling liquid is added evidence that it is not the ordinary MQM compound. Though the coordinates of the carbon atoms cannot be obtained from the X-ray data there can be no question as to the positions of the pla'tinum atoms. In both compounds, there are sets of four platinum atoms forming regular tetrahedra, each set compact and well separated from neighboring sets. The platinum-platinum distances are too great to permit strong platinum-platinum b n d ing, but too small for the platinum atoms within the group of four to belong to different molecules. This practically demands that the molecules be tetramers with the platinum bonded through other atoms. In trimethylplatinum chloride it is quite easily established from the X-ray evidence alone that the chlorine atoms are in positions to do this. Though it seems objectionable to bond a methyl group to three platinum atoms there is no other alternative in tetramethylplatinum. This need not mean that carbon violates the octet rule, but it may indicate that the two electrons involved in bonding three platinum atoms to a carbon atom resonate among three possible bonds. In trimethylplatinum chloride the platinum-chlorine distance, 2.48 A., appears too small for ionic bonding, and it is very doubtful if the unusual methyl bonding can be attributed to a methyl ion. If the structure described above is accepted, it seems necessary to bond the other methyls to platinum, and thereby to complete the usual octahedral bonding to tetravalent platinum. The packing of the compound is very satisfactory on t h i s basis. The extremely high temperature factors (Tables I and 11)are in accord with the hydrocarbon-like packing of the tetramers. The bridging of platinum to platinum through chlorine in trimethylplatinum chloride is not unlike the reported bridging of aluminum to aluminum through halogens in the aluminum halide dimers.11*12It has been suggested that methyls bridge the aluminums in the aluminum methyl dimer. This structure is not in agreement with (11) K.J. Palmer and Norman Elliott, Tms JOURNAL, 60, 1862 (1938). (12) L. 0. Brockaay and N. R. Davidson, ibid., 0 , 3 2 8 7 (1941).

July, 1947

CLASSIFICATION OF OXIDATION

REACTIONS ON BASIS OF SEMIQUINONE THEORY 1567

published electron diffraction investigations,12Ja but later, as yet uncompleted, studies in these Laboratories defhitely &ow that the published tmxhskms are an~&is€mtory.~~ Tetrsmethylplatirrarm then pmvkks the first established example of a methyl group barnded to more than one other atom. Acknowledgment.-The authors are indebted to Professor Henry Gilman for supplying crystals of tetramethylplatinurn, and for helpful advice and discussions regarding the chemistry of the platinum alkyls and their derivatives. They benefited also from frequent consultations with Professor Linus Pauling. summary The crystal structures of trimethylplatinum chloride and tetramethylplatinum have been determined. The crystals are isomorphic, with (13) N. R. Davidson, J. A. C Hugill, H. A. Skinner, and L. E Sutton, Trans. Faraday Soc., 86, 1212 (1940). (14) Private communication from Professor Verner F. H. Schomaker.

space group TdLI@??%. The edge of the smallest c& unit is 10.55 A. for ti cI&wi.de and 10.145 A. for tetrame&y1pla~nam. ”irnethylplatinurn chloride is a tetrarnw with platinwn atoms and chlorine atwrrrms at & m a t e corners of a distorted cube. Bonded to each platinum are three methyl groups. Tetramethflplatinum is similar, with methyls replacigg the cfilorines. Two of these tetramers, of point symmetry Td, compose the bodycentered unit. Interatomic distances involving carbon atoms had to be.assumed. The distance Pt-Cl = 2.48 k., Cl-Cl = 3.28 k., L Pt-Cl-l?t = 99’. The shortest distance Pt-Pt in PtMesCl is 3.73 A.; in P ~ M Qit is 3.44 k . The M e M e distances between adjacent tetramers are slightly more than 4 A. Tetramethylplatinum provides the first case where a methyl group is bonded to more than one other atom. Neither compound shows the expected d2sp bonds to platinum. PASADENA, CALIFORNIA

RECEIVEDJANUARY

29, 1947

[ COM~UUNICATION NO. 1137 FROM THE KODAK RESEARCH LABORATORIES]

Oxidation Processes. XVIII.’

A Classification of Reactions on the Basis of the Semiquinone Theory

BY J. E. LUVALLEAND A. WEISSBERGER The preceding papers of this series dealt with the oxidation of a-ketols (-CHOHCO-), aaminoketones (-CHNH*CO), enediols (-COH= COH-), and hydroquinones by oxygen and, in some cases, by cupric salts. Kinetic experiments yielded rate laws which were interpreted in terms of reactive species and reaction mechanisms. For instance, it was shown that in these reactions, the mono- and the divalent anions of the various compounds are more reactive than the neutral molecules and that the monomeric products of univalent oxidation, the semiquinones, occupy key positions in the reactions.lV2 The present paper derives the observed rate laws in a systematic way, making full use of the equilibria in two-step oxidations and of the concept of the steady state. This treatment gives a classification of the earlier r e ~ u l t s ~and + ~ Jfurnishes a ready means for the interpretation of more recent data on aromatic amine^.^ The existence of semiquinones, i. e., of rather stable free radicals which axe intermediate in their state of oxidation between p- or o-diaminobenzenes and quinonediimines, between hydro(1) Part XVII, Weissberger and LuValle, THIS JOWENAL, 66, 700 (1944). (b) Weiss(2) (a) James and Wekberger, ibid., (10, 98 (1938); berger, LuVallC and Thomas, {bid,, 66, 1934 (1943). (3) (a) James, h e l l and Weissberger, ibid., 60, 2084 (1938); (b) Kornfeld and Wekberger, ibid.. 61, 360 (1939). (4) LuValle and Weissberger, to be published.

quinones and quinones, etc., was first suggested .by Hantz~&.~They were thoroughly studied and discussed by Weitz.@ Elem’ m d Michaelis8 showed independently that b i d a t oxidations and reductions proceed in univalent steps and that the intermediate semiquinones are in equilibrium with the reduced and the oxidized forms (dismutation) and with their dimers. Both equilibria are in accord with the radical character of the semiquinones. The values of the respective equilibrium constants are functions of the pH,8p0 because dimerization and dismutation of the ions are inhibited by electrostatic repulsion.6*10However, the stability of the semiquinones appears to be primarily caused by resonance.11 The resonance is smaller if the structures are non-equivaknt. (5) Hantzsch, Ber., 4b,519 (lQl6); 64, 1278 (1921). (6) Weitz, Z.Elckklrochcm., 84, 538 (1928). (7) Elema, Rcc. frav. chim., 60, 807 (1931); 64, 569 (1933). (8) (a) Friedheim and Michaelis, J . B i d . Chsm., 91, 355 (1931); (b) Michaelis, ibid., 94, 211 (1931); (c) Michaelis, THIS JOURNAL, 68, 2953 (1931); (d) Michaelis, Chum. Rev., 16, 243 (1935); (e) Michaelis and Schubert, ibid., 49, 437 (1938); (f) Michaelis and Smythe, Ann. Rcu. Biochcm., 7‘, 1 (1938); (9) Michaelis, Ann. N . Y . Acad. Sci., 40,39 (1940). (9) Michaelis, Granick and Schubert, THIS JOURNAL, 68, 351 (1941); Michaelis and Granick, ibid., 68, 1636 (1941); Schubert, Ann. N . Y . Acad. Sci., 40, 111 (194a). (le) Weiss, Trans. Faraday Soc., a,116 (1946). (11) Pauling, “The Nature of the Chemical Bend,” Cornell University Press, Ithaca, 1940, Chap. IV; Wheland, “The Theory of Resonance,” John Wiley and Sons, New York, 1944.