α-agostic interactions in Cp*W(NO)(CH2CMe3)2 and related nitrosyl complexes

The solid-state molecular structure of Cp*W(NO)(CH2CMe3)(2) (1) has been shown to contain one "strongly agostic" and one "weakly agostic" methylene hydrogen atom by a neutron diffraction analysis at 120 K and by an X-ray diffraction analysis at -100 degreesC. The X-ray diffraction analysis of Cp*W(NO)(CH2SiMe3)(2) (2) at -20 degreesC reveals that its solid-state molecular structure possesses a crystallographically imposed mirror plane. Consequently, only the average existence of a relatively strong C-(HM)-M-... interaction for each hydrocarbyl ligand in 2 in the solid state can be established. The H-1 and gated C-13{H-1} NMR spectra of representative Cp'M(NO)(R)(R') (M = Mo, W; R = hydrocarbyl, R' = hydrocarbyl, halide, amide) complexes exhibit spectral parameters for the alpha -H and alpha -C atoms (i.e., delta (H), delta (C), Delta delta (H), DeltaJ(HC), and J(HW)) that provide evidence for the presence of a-agostic interactions in the molecular structures of these complexes in solution. The picture that has emerged from these investigations is that these complexes do adopt an a-agostic structure both in solution and in the solid state when secondary interactions such as pi -electron donation are weak (as in Cp*W(NO)(CH2CMe3)Cl), competitive (as in Cp*W(NO)(CH2CMe3)(CH2Ph)), or not possible (as in Cp*W(NO)(hydrocarbyl)(2) complexes generally). The bis(hydrocarbyl) complexes are probably stereochemically nonrigid in solution, as they appear to interconvert between the two limiting structures having one strongly agostic and one weakly agostic hydrocarbyl ligand. The neopentyl ligand forms alpha -agostic linkages with the metal centers in all neopentyl complexes studied, and these agostic interactions appear to be independent of the nature of the metal or the ancillary cyclopentadienyl ligand. Interestingly, the presence of these groundstate agostic interactions does not correlate with the tendency of the various compounds to undergo intramolecular alpha -H abstraction reactions and form reactive alkylidene complexes.