Effects of the niobium(V) center on the energetics of ligand-centered proton and hydrogen atom transfer reactions in acyl and alkoxide complexes

Cationic niobium eta(2)-acyl (1) and alkoxide (3) compounds are relatively strong C-H acids which give stable eta(2)-ketene (2) and eta(2)-formaldehyde (4) complexes upon deprotonation. Determinations of the C-H bond dissociation enthalpies (BDEs) show that 2(.+) gains little stabilization from radical delocalization but that generation of a Nb-C sigma bond facilitates the homolytic conversion of 3 to 4(.+).