Theoretical Studies of the Reactivity of Cyclopentadienyl Nitrosyl Alkyl Species of Molybdenum and Tungsten

The reactivity differences observed experimentally for Cp*W(NO)(CH 2 CMe 3 ) 2 and CpMo(NO)(CH 2 CMe 3 ) 2 have been investigated using density functional theory (DFT) techniques. The reactions of the CpW(NO)(CH 2 ) model complex with NH 3 and CH 4 are more exothermic and have lower activation barriers than the corresponding processes for CpMo(NO)(CH 2 ). The η 2 (C,H) methane complex CpM(NO)(CH 2 )(CH 4 ) (M = Mo, W) can undergo two competitive processes: C−H activation to afford CpM(NO)(CH 3 ) 2 or loss of methane. The relative barrier heights are almost identical for M = W, whereas the formation of CpM(NO)(CH 3 ) 2 is significantly disfavored for M = Mo. The activation of C−H and N−H bonds proceeds via a direct, metal-assisted 1,2 addition across the M CH 2 bond without a distinct carbene−hydride intermediate. The relative stability order of the products can be rationalized in part through metal−ligand π-bonding considerations. The relative orbital energies, which are consistently lower for Mo than for W, can be used to rationalize various reactivity differences observed experimentally for the CpM(NO)(alkyl) 2 species, including the selective coordination of small molecules. The absence of sufficient steric bulk in the model complexes likely contributes to the overestimation of the calculated activation energies and the discrepancy between some of the calculated and experimental structural parameters.