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{(SiMe₂)···Do···(SiMe₂)} (Cp = η-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)₃Me and HSiMe₂SiMe₂Do. The X-ray crystal structures of 1a, 1b, and 2 revealed that the M−Si bonds (2.4795(9) and 2.4804(9) Å for 1a, 2.4996(9) and 2.5008(9) Å for 1b, and 2.355(2) Å for 2) are significantly shorter than those of structurally similar silylmolybdenum and -chromium complexes, while the Si−O bonds in 1a (1.782(2) and 1.788(3) Å) and 2 (1.788(3) Å) and the Si−N bonds in 1b (1.933(2) and 1.923(3) Å) 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 shift of the ²⁹Si NMR signals (1a, δ = 117.6 ppm; 1b, δ = 80.5 ppm; 2, δ = 128.6 ppm) compared to those of structurally similar silylmolybdenum and -chromium complexes. Complexes 1a, 1b, and 2 showed fluxional behavior due to silylene−methyl group exchange and, in the case of 1b, N−Et group exchange. A mechanism involving the generation of a base-free silyl(silylene) complex as the key intermediate is proposed to explain the fluxional process.