Electrochemical reduction and reoxidation accompanied by reversible geometric isomerization. Electrochemistry of bis(dicarbonyl-.mu.-di-tert- butylphosphidorhodium). Isolation and x-ray crystal structure of bis(tetra-n- butylammonium) bis(dicarbonyl-.mu.-di-tert-butylphosphidorhodate)

Electrochemical studies on [ R ~ ( ~ ~ B U , P ) ( C O ) ~ ] ~ have shown that upon reduction in THF it undergoes an ECE-type mechanism, a chemical reaction coupled between two electron-transfer reactions, to form a dianion. The chemical step is a geometric isomerization, producing an isomer which is more easily reduced than the parent compound. Oxidation of the dianion back to the neutral compound occurs by two one-electron oxidations followed by the reverse isomerization. Spectroscopic and crystallographic data, as well as digital simulations of the voltammetric responses, are presented to support the proposed mechanism. The dianion, [Rh(p-t-B~,P)(C0)~1~-, was isolated as the N(n-Bu)4+ salt, and its structure in the solid state has been determined by a single-crystal X-ray diffraction study. The two Rh atoms of the dianion have pseudotetrahedral geometries and a Rh-Rh distance of 2.840 (1) A which is consistent with a metal-metal bond of order one. Crystal data: C52H108N204P2Rh2, M = 1093.22, monoclinic, P2,/c, a = 14.138 (1) A, b = 14.370 (4) A, c = 21.127 (1) A, p = 107.367 (3)", U = 4096.8 (1) A3, D, = 1.688 g ~ m ~ , 2 = 2, X(Mo Ka) = 0.71069 A, ~ ( M o Ka) = 9.27 cm-I, final R = 0.0688, R, = 0.0945 from 2729 observed reflections ( I > 2 4 ) , 4463 measured. Geometric rearrangement or isomerization upon either oxidation or reduction has been observed for a number of organic and inorganic systems. Previous studies from this laboratory and others have shown that organic molecules containing either steric strain or activated functionalities may rearrange following electron transfer.' Bond, Geiger, and others have observed similar effects with organometallic complexes, especially those containing diphenylphosphido ligands., There is currently considerable interest in the chemistry of phosphidoand arsenido-bridged complexes of the transition metal^.^ As part of a broad program designed to study the steric ( 1 ) (a) Yeh, L.-S. R.; Bard,A. J. J . Electrochem. Soc. 1973,124, 189-195. (b) Bard, A. J.; hglisi, V. J.; Kenkel, J. V.; Lomax, A. Discuss. Faraday Soc. 1973,56, 353-366. (c) Yeh, L.-S. R.; Bard, A. J. J. Electroanal. Chem. 1976, 70, 157-169. (d) Phelps, J.; Bard, A. J. Ibid. 1976, 68, 313-335. (e ) Yeh, L .4 . R.; Bard, A. J. Ibid. 1977, 81, 333-338. (f) Chien, C. K.; Wang, H. C.; Szwarz, M.; Bard, A. J.; Itaya, K. J . Am. Chem. SOC. 1980, 102, 3100-3104. (g) Olsen, B. A,; Evans, D. H. Ibid. 1981, 103, 839-843. (h) Ahlberg, E.; Hammerich, 0.; Parker, V. D. Ibid. 1981, 103, 844-849. (i) Neta, P.; Evans, D. H. Ibid. 1981, 103, 7041-7045. (j) Evans, D. H.; Busch, R. W. Ibid. 1982, 104, 5057-5062. (k) Olsen, B. A,; Evans, D. H.; Agranat, I. J . Electroanal. Chem. 1982, 136, 139-148. (I) Evans, D. H.; Xie, N. J . Am. Chem. SOC. 1983, 105, 315-320. (2) (a) Wimmer, F. L.; Snow, M. R.; Bond, A. M. Inorg. Chem. 1974,13, 1617-1623. (b) Bond, A. M.; Colton, R.; Jackowski, J. J. Ibid. 1975, 14, 274-278. (c) Bond, A. M.; Colton, R.; McCormick, M. J. Ibid. 1977, 16, 155-159. (d) Bond, A. M.; Grabaric, B. S.; Grabaric, 2. Ibid. 1978, 17, 1013-1018. ( e ) Bond, A. M.; Colton, R.; Jackowski, J. J. Ibid. 1978, 17, 2153-2160. (f) Bond, A. M.; Colton, R.; McDonald, M. E. Ibid. 1978, 17, 2842-2847. (9) Bond, A. M.; Keene, F. R.; Rumble, N. W.; Searle, G. H.; Snow, M. R. Ibid. 1978,17,2847-2853. (h) Bond, A. M.; Darensbourg, D. J.; Mocellin, E.; Stewart, B. J. Ibid. 1981, 19,6737-6732. (i) Takvoryan, K.; Katovic, V.; Lovecchio, F. L.; Gore, E. S.; Anderson, L. B.; Busch, D. H. J . Am. Chem. Sot. 1974, 96, 731-742. (j) Holloway, J.; Bowden, W.; Geiger, W. E. Ibid. 1977,99,7089-7090. (k) Holloway, J.; Geiger, W. E. Ibid. 1979, 101, 2038-2044. (I) Moraczewski, J.; Geiger, W. E. Ibid. 1979, 101, 3407-3408. (m) Tulyathan, B.; Geiger, W. E. Ibid. 1980,109, 325-331. (n) Moraczewski, J.; Geiger, W. E. J . Am. Chem. SOC. 1981, 103, 4779-4787. (0 ) Albright, T. A.; Geiger, W. E.; Moraczewski, J.; Tulyathan, B. Ibid. 1981, 103,4787-4794. (p) Arewgoda, C. M.; Robinson, B, H.; Simpson, J. J . Chem. SOC., Chem. Commun. 1982, 92, 284-285. (3) For leading references see: (a) Breen, M. J.; Geoffroy, G. L.; Rheingold, A. L J . Am. Chem. Sot. 1983, 105, 1069-1070. (b) Fultz, W. C.; Rheingold, A. L.; Kreter, P. E.; Meek, D. W. Inorg. Chem. 1983, 22, 860-863. (c) Carty, A. J. Adu. Chem. Ser. 1982, No. 196, 163. (d) Braunstein, P.; Mott, D.; Fars, 0.; Louer, M.; Grandjean, D.; Fischer, J.; Mitschler, A. J . Organomet. Chem. 1981, 213, 79-92. (e ) Yu, Y.-F.; Gallucci, J.; Wojcicki, A. J . Am. Chem. Soc. 1983, 105, 4826-4828. (f) Zolk, R.; Werner, H. J . Organomet. Chem. 1983, 252, C53-C56. (g) Muller, M.; Vahrenkamp, H. Chem. Ber. 1983, 116, 2322-2336. 0002-7863/85/1507-0888$01.50/0 and electronic effects of phosphido complexes, we have investigated the use of sterically demanding (bulky) alkyl groups attached to pho~phorus"'~ or arsenic.14 Our initial studies have focused on the use of the di-tert-butylphosphido group (t-Bu,P-). For rhodium a number of phosphido-bridged complexes have been reported.lS We recently described the unusual dinuclear Rh(1) system based on two isomers of the di-tert-butylphosphido-bridged complex [Rh(p-t-Bu,P)(CO),], (see Scheme I). The interesting feature of this system is a facile reversible metal-metal bond cleavage accompanied only by a geometric isomerization. We have noted that for dinuclear phosphidobridged complexes of the Co group with roughly planar M2P2 central cores, three geometric isomers have been observed-A, B, and C (see Scheme II).839*11 On the basis of bond length considerations, the geometry about each metal atom (either tetrahedral or planar) may be correlated with metal-metal bonds of order 0, 1, and 2 in A, B, and C, respectively. By reaction with PMe3, isomers 1 and 2 may be converted into complex 3, con(4) Jones, R. A.; Stuart, A. L.; A t w d , J. L.; Hunter, W. E.; Rogers, R. (5) Jones, R. A,; Wright, T. C.; Atwood, J. L.; Hunter, W. E. Inorg. Chem. (6) Jones, R. A.; Wright, T. C.; Atwood, J. L.; Hunter, W. E. Organo(7 ) Jones, R. A,; Stuart, A. L.; Atwood, J. L.; Hunter, W. E. Organo(8) Jones, R. A.; Stuart, A. L.; Atwood, J. L.; Hunter, W. E. Organo(9) Jones, R. A.; Norman, N. C.; Seeberger, M. H.; Atwood, J. L.; Hunter, (10) Jones, R. A.; Lasch, J. G.; Normae, N. C.; Wright, T. C. J . Am. (11) Jones, R. A.; Wright, T. C. Organometallics 1983, 2, 1842-1845. (12) Jones, R. A.; Lasch, J. G.; Norman, N. C.; Stuart, A. L.; Wright, T. (13) Jones, R. A.; Stuart, A. L.; Wright, T. C. J . Am. Chem. SOC. 1983, (14) Jones, R. A,; Whittlesey, B. R. Organometallics 1984, 3, 469-475. (15) (a) Jamerson, J. D.; Pruett, R. L.; Billig, E.; Fiato, F. A. J . Organomet. Chem. 1980, 193, C43-C46. (b) Billig, E.; Jamerson, J. D.; Pruett, R. L.; Ibid. 1980, 192, C49-C51. (c) Haines, R. J.; Stem N. D. C. T.; English, R. B. Ibid. 1981, 209, C34-C36. (d) Kreter, P. E.; Meek, D. W.; Christoph, G. G. Ibid. 1980, 188, C27-C30. ( e ) Meek, D. W.; Kreter, P. E.; Christoph, G. G. Ibid. 1982, 231, C53-C55. (f) Kreter, P. E.; Meek, D. W. Inorg. Chem. 1983, 22, 319-326. (9) Burkhardt, E. W.; Mercer, W. C.; Geoffroy, G. L.; Rheingold, A. L.; Fultz, W. C. J . Chem. SOC., Chem. Commun. 1983, 1251-1252. D. Organometallics 1982, 1 , 1721-1723.