alpha-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 sh own to contain one "strongly agostic" and one "weakly agostic" methylene hy drogen atom by a neutron diffraction analysis at 120 K and by an X-ray diff raction analysis at -100 degreesC. The X-ray diffraction analysis of Cp*W(N O)(CH2SiMe3)(2) (2) at -20 degreesC reveals that its solid-state molecular structure possesses a crystallographically imposed mirror plane. Consequent ly, only the average existence of a relatively strong C-(HM)-M-... interact ion 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 exh ibit 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 evidenc e for the presence of a-agostic interactions in the molecular structures of these complexes in solution. The picture that has emerged from these inves tigations is that these complexes do adopt an a-agostic structure both in s olution and in the solid state when secondary interactions such as pi -elec tron 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) comp lexes generally). The bis(hydrocarbyl) complexes are probably stereochemica lly nonrigid in solution, as they appear to interconvert between the two li miting structures having one strongly agostic and one weakly agostic hydroc arbyl ligand. The neopentyl ligand forms alpha -agostic linkages with the m etal centers in all neopentyl complexes studied, and these agostic interact ions appear to be independent of the nature of the metal or the ancillary c yclopentadienyl ligand. Interestingly, the presence of these groundstate ag ostic interactions does not correlate with the tendency of the various comp ounds to undergo intramolecular alpha -H abstraction reactions and form rea ctive alkylidene complexes.