Structure-reactivity correlations in new W(IV) and Nb(IV) silicon-bridged ansa-metallocene hydrides

W(IV) and Nb(IV) derivatives: [W(L1)MeH] (1), [W(L1)MeH] (2) and [W(L1)O] (3), [W(L2)Cl-2](4), [W(L2)H-2](5), [W(L2)H-3][BF4] (6), [Nb(L2)Cl-2](7) and [Nb(L2)(eta(2)-BH4)](8), where L1 = [(eta-C5H4)(2)SiMe2](2-) and L2 = [(eta-C5H3TMS)(2)SiMe2](2-), have been prepared and characterised. The crystal structures of compounds 5 and 8 have been determined. Thermodynamic parameters for the elimination of methane from compound 2 have been evaluated based on H-1 NMR studies in C6D6. Trimethylsilyl substituents on the cyclopentadienyl rings appear to moderate the structural and dynamic properties of the silicon-bridged ansa-tungstenocenes.