ELECTRON-TRANSFER IN GAS-PHASE LIGAND-SWITCHING REACTIONS

Bimolecular rate constants for the gas-phase ion-molecule reactions of V(CO)5- with a variety of molecules have been obtained at thermal energies by using Fourier transform ion cyclotron resonance spectroscopy. Three distinct reaction mechanisms are observed. For the first type, ligand displacement of CO by various neutrals is observed. For a variety of ligands a correlation is observed between the reaction efficiency and the ligand electron affinity. For all reactive ligands, extensive loss of carbonyls is observed. A mechanism is proposed which involves a 1-e- transfer from V(CO)5- to the incoming ligand, facilitated by the initial ion-dipole complexation energy. Simple ligand field orbital diagrams indicate the acceptor orbital for V(CO)5- is half-filled. Thus, 2-e- donors cannot coordinate to the metal center without electron transfer. The radicals NO. and (tert-butyl)2NO., which can act as 1-e- donors to the half-filled acceptor orbitals, react rapidly to displace multiple CO from V(CO)5-. NO. displaces CO at approximately 16% of the collision frequency, while (t-C4H9)2NO. reacts at < 1% of the collision frequency. The difference in reaction efficiency for the radicals is most likely due to a steric effect introduced by the large tert-butyl groups. The ligand displacement results suggest V(CO)5- is a triplet in the ground state. Most molecules that undergo primary reaction with V(CO)5- also undergo secondary reaction via further displacement of carbonyl ligands. The second type of reaction studied was chlorine transfer from chloromethanes to generate predominantly VCl(CO)x- (x = 3-5). The rate constants correlate with the C-Cl bond dissociation energies, suggesting initial insertion into the C-Cl bond or a direct chlorine atom transfer mechanism. HCl addition to the metal center becomes competitive as the rate for chlorine transfer decreases. The third type of reaction studied involved oxidative addition/reductive elimination. V(CO)5- reacts with CH3NH2 to generate V(CO)4(CH2NH)-. Dehydrogenation is only observed with monomethyl-substituted organic molecules (i.e. CH3OH and CH3NH2) suggesting a steric effect in one of the intermediates.