Interaction of a Coordinatively Unsaturated Transition-Metal Center with Small Molecules

DFT calculations were performed in order to investigate the interactions between coordinatively unsaturated sites on W(CO)5 and C2H4 as a typical π-acceptor and NH3 as a typical σ-donor. B3LYP-based DFT calculations with ECPs (LANL2DZ) on W and 6-311G(d,p) all-electron basis sets on H, C, N, and O reproduced the characteristic shifts in vibrational frequency of carbonyls that have been experimentally observed for the successive processes W(CO)6 (- CO) -> W(CO)5 (+ L) -> W(CO)5L (L = C2H4 and NH3). Based on the calculations, C2H4 coordinates to the coordinatively unsaturated site of W atom in a side-on fashion with a 23.1 kcal mol-1 bond dissociation enthalpy (BDE), where the center of the C=C double bond was 2.39 Å from W atom, and NH3 coordinates with a BDE of 31.9 kcal mol-1 and a distance of 2.37 Å. As well, the C=C bond length (1.38 Å) in C2H4 was found to be slightly longer than that of free C2H4 (1.33 Å).