Synthesis, crystal structures, and fluxional behavior of donor-bridged bis(silylene)molybdenum and -chromium complexes

Donor-bridged bis(silylene)molybdenum and -chromium complexes CpM(CO)(2){(S iMe2)... Do ...(SiMe2)} (Cp = eta-C5H5; M = Mo, Do = OMe (1a); M = Mo, Do = NEt2 (1b); M = Cr, Do = OMe (2)) were synthesized by photolysis of a C6D6 solution containing CpM(CO)(3)Me and HSiMe(2)SiMe(2)Do. The X-ray crystal s tructures of 1a, Ib, and 2 revealed that the M-Si bonds (2.4795(9) and 2.48 04(9) Angstrom for 1a, 2.4996(9) and 2.5408(9) Angstrom for 1b, and 2.355(2 ) Angstrom for 2) are significantly shorter than those of structurally simi lar silylmolybdenum and -chromium complexes, while the Si-O bonds in 1a (1. 782(2) and 1.788(3) Angstrom) and 2 (1.188(3) Angstrom) and the Si-N bonds in 1b (1.933(2) and 1.923(3) Angstrom) are much longer than usual Si-O and Si-N single bonds. These structural data indicate that the M-Si bonds bear partial double-bond character, whereas the Si-O and Si-N bonds are regarded as a hybrid of covalent bonding and dative bonding. The unsaturated nature of the metal-silicon bonds is also shown by the significant downfield shif t of the Si-29 NMR signals (1a, delta = 117.6 ppm; 1b, delta = 80.5 ppm; 2, delta = 128.6 ppm) compared to those of structurally similar silylmolybden um and -chromium complexes. Complexes la, Ib, and 2 showed fluxional behavi or due to silylene-methyl group exchange and, in the case of 1b, N-Et group exchanger A mechanism involving the generation of a base-free silyl(silyle ne) complex as the key intermediate is proposed to explain the fluxional pr ocess.