Oxidative Additions of Coordinated Ligands at Unsaturated Molybdenum and Tungsten Diphosphine-Bridged Carbonyl Dimers. 3. Decarbonylation Reactions of [MoW(η 5 -C 5 H 5 ) 2 (CO) 4 (μ-Ph 2 PCH 2 PPh 2 )] Celedonio Alvarez, M. Esther Garcı ´a, Vı ´ctor Riera, and Miguel A. Ruiz* Departamento de Quı ´mica Orga ´ nica e Inorga ´ nica/IUQOEM, Universidad de Oviedo, E-33071 Oviedo, Spain Received October 31, 1996 X Decarbonylation of the heterometallic title complex [MoWCp 2 (CO) 4 (μ-dppm)] (Cp ) η 5 - C 5 H 5 ; dppm ) Ph 2 PCH 2 PPh 2 ) in refluxing tetrahydrofuran leads to the phosphido complex [MoWCp 2 (μ-CH 2 PPh 2 )(μ-PPh 2 )(CO) 2 ], presumably via the tricarbonylic species [MoWCp 2 (μ- CH 2 PPh 2 )(μ-PPh 2 )(μ-CO)(CO) 2 ]. The latter is an unstable compound which can be generated upon reaction of the former with CO and also contains the phosphinomethyl ligand C-bonded to the tungsten atom. In contrast, photolytic decarbonylation of the title complex at -10 °C leads reversibly to the hydrido compound [MoW(μ-η 1 :η 5 -C 5 H 4 )Cp(μ-H)(CO) 3 (μ-dppm)], in which the cyclopentadienylidene ligand is specifically η 5 -bonded to tungsten and η 1 -bonded to molybdenum. Further photolysis of this complex at 10 °C leads to the triply bonded dimer [MoWCp 2 (CO) 2 (μ-dppm)] with concomitant regeneration of a H-C (cyclopentadienyl) bond. The latter species reacts with t BuCN to give [MoWCp 2 (μ-η 1 :η 2 -CN t Bu)(CO) 2 (μ-dppm)], in which the isocyanide ligand is specifically η 1 -bonded to tungsten and η 2 -bonded to molybdenum. In order to account for the metal selectivity observed in the above species, two different intermediates, having either μ-η 1 -CO or μ-η 1 :η 2 -CO ligands, are thought to be involved in the decarbonylation reactions of the title compound. Introduction In the previous parts of this series 1,2 we have shown that the unsaturated species generated through ejection of carbon monoxide from the single-metal-metal- bonded dimers [M 2 Cp 2 (CO) 4 (μ-dppm)] (Cp ) η 5 -C 5 H 5 ; dppm ) Ph 2 PCH 2 PPh 2 ;M ) W, 1 Mo 2 ) are able to activate C-H or P-C (sp 3 ) bonds in the coordinated cyclopentadienyl or diphosphine ligands, respectively. The results are strongly dependent on the metal. Thus, when M ) Mo, only P-C bond activation is observed, which leads irreversibly to the phosphido-phosphinom- ethyl complex [Mo 2 Cp 2 (μ-CH 2 PPh 2 )(μ-PPh 2 )(CO) 2 ]. In contrast, when M ) W a reversible C-H bond activation occurs, which gives the hydrido cyclopentadienylidene compound [W 2 (μ-η 1 : η 5 -C 5 H 4 )Cp(μ-H)(CO) 3 (μ-dppm)]. From the above studies we could not give a fully satisfactory explanation for this striking difference in the chemical behavior of the dimolybdenum and ditungsten systems. Thus, while the behavior of the dimolybdenum system could be in part due to the low thermodynamic stability of the cyclopentadienylidene ligand when bonded to this metal, 3 it was not clear why the P-C cleavage of the backbone of the dppm ligand occurs readily in the dimolybdenum complex but is almost absent in the ditungsten analogue under all decarbonylation condi- tions examined (thermal or photochemical). Because of the fact that both P-C 4 and C-H 5 bond cleavage reactions are relevant processes in the chemistry of organometallic compounds, we were interested in gain- ing more insight about the influence of the metal on these activation processes. We therefore decided to study the decarbonylation reactions of the mixed-metal dimer [MoWCp 2 (CO) 4 (μ-dppm)] (1). Results and Discussion Synthesis and Structure of [MoWCp 2 (CO) 4 (μ- dppm)] (1). As found for its homonuclear analogues, * To whom correspondence should be addressed. E-mail: mara@ sauron.quimica.uniovi.es. X Abstract published in Advance ACS Abstracts, March 1, 1997. (1) (a) Part 1: Alvarez, M. A.; Garcı ´a, M. E.; Riera, V.; Ruiz, M. A.; Falvello, L. R.; Bois, C. Organometallics 1997, 16, 354. (b) Alvarez, M. A.; Garcı ´a, M. E.; Riera, V.; Ruiz, M. A.; Bois, C.; Jeannin, Y. J. Am. Chem. Soc. 1993, 115, 3786. (2) (a) Part 2: Garcı ´a, G.; Garcı ´a, M. E.; Melo ´n, S.; Riera, V.; Ruiz, M. A.; Villafan ˜ e, F. Organometallics 1997, 16, 624. (b) Riera, V.; Ruiz, M. A.; Villafan ˜ e, F.; Bois, C.; Jeannin, Y. J. Organomet. Chem. 1989, 375, C23. (3) Alvarez, M. A.; Garcı ´a, M. E.; Riera, V.; Ruiz, M. A.; Bois, C.; Jeannin, Y. J. Am. Chem. Soc. 1995, 117, 1324. (4) (a) Chaudret, B.; Delavaux, B.; Poilblanc, R. Coord. Chem. Rev. 1988, 86, 191. (b) Garrou, P. E. Chem. Rev. 1985, 85, 171. (5) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987; p 295. (b) Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1993, 32, 789. (c) Ryabov, A. D. Chem. Rev. 1990, 90, 403. (d) Jones, W. D.; Feher, F. J. Acc. Chem. Res. 1989, 22, 91. (e) Crabtree, R. H. Chem. Rev. 1985, 85, 245. (f) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983, 250, 395. Chart 1 1378 Organometallics 1997, 16, 1378-1383 S0276-7333(96)00920-X CCC: $14.00 © 1997 American Chemical Society