PHOSPHINE-INDUCED SILYL MIGRATION IN HETERODINUCLEAR ACYL COMPLEXES 3)FE(MU-SI(OME)(2)(OME))(MU-DPPM)PT(C(O)R)](FE-PT) - LEADING TO MU-SILOXYCARBENE COMPLEXES - CRYSTAL-STRUCTURE OF (OC)(3)FE(MU-C(ET)OSI(OME)(3))(MU-DPPM)PT(PPH(3))]

Carbon monoxide reacts with the heterodinuclear complexes [(OC)(3)Fe{m u-Si(OMe)(2)(OMe)}-(mu-dppm)Pt(R)] (R = Me, Et) by opening the alkoxys ilyl bridge, followed by insertion into the cis Pt-alkyl bond to yield the bimetallic acyl complexes [(OC)(3)Fe{mu-Si(OMe)(2)(OMe)}(mu-dppm) Pt{C(O)R}] (1a,b) in which the acyl ligand is trans with respect to th e metal-metal bond. The driving force for the isomerization of the acy l ligand comes from the tendency to restore the mu-Si-O bridge. Surpri singly, these acyl complexes were not obtained in the reaction of the metalate K[Fe{Si(OMe)(3)}(CO)(3)(eta(1)-dppm)] with trans-[PtCl{C(O)R} (PPh(3))(2)]; instead the stable mu-siloxycarbene complexes [(OC)(3)Fe {mu-C(R)OSi(OMe)(3)}(mu-dppm)Pt(PPh(3))] (2a, R = Me; 2b, R = Et) were isolated in ca. 75% yield. Similar complexes were obtained in almost quantitative yields from the reaction of 1a with phosphine or phosphit e ligands. The migration of the silyl ligand from iron to the oxygen a tom of an acyl ligand bound to the neighboring platinum atom involves silicon-oxygen (acyl) and iron-carbon (acyl) bond formation. The C-O u nit found in the carbene ligand stems from the acyl group (C-13 labeli ng experiment). The phosphorus donor is believed to make the acyl oxyg en more electron rich, thus favoring Si migration, and stabilizes the acyl ligand cis with respect to the metal-metal bond long enough to al low Si migration to occur. This silyl migration could not be induced b y isonitrile ligands although the mu-siloxycarbene complexes [(OC)(3)F e{mu-C(R)OSi-(OMe)(3)}(mu-dppm)Pt(C=NR')] (4a, R = Me, R' = t-Bu; 4b, R = Et, R' = 2,5-xylyl) were isolated from the reactions of 2a,b with the corresponding isonitrile. Cleavage of the C-O bond of 2b afforded the cationic mu-carbyne complex [(OC)(3)Fe(mu-CEt)(mu-dppm)Pt(PPh(3))] [CF3SO3] (5). The crystal structure of 2b . 2C(7)H(8) has been determi ned by X-ray diffraction and appears to be the first one for a polymet allic siloxycarbene complex: monoclinic space group P2(1)/c with Z = 4 in a unit cell of dimensions a = 11.014(1), b = 21.148(1), c = 26.063 (1) Angstrom, beta = 94.57(2)degrees. The structure has been solved fr om diffractometer data by Patterson and Fourier methods and refined by full-matrix least squares on the basis of 8375 observed reflections(I > 3 sigma(I)) to R(F) and R(w)(F) values of 0.039 and 0.055.