Guided Ion Beam Studies of the Energetics of Organometallic Species

Abstract

Dissociation energies for a variety of bonds between transition metals and simple ligands comprised of hydrogen, carbon, nitrogen, and oxygen have been measured by using the technique of guided ion beam mass spectrometry. For species that form covalent bonds with the metals, the bond energies are found to correlate with the energy necessary to promote the bare metal atom or ion into an electron configuration suitable for bonding. The trends in the bond energies of M+ to isoelectronic series of ligands, CH3, NH2 and OH or CH2, NH and 0, are used to quantify the contributions of covalent and dative bonding for these ligands. The strength of the dative interactions is found to depend on the number of electron lone-pairs on the ligands and the number of empty or half-filled d orbitals on the metal. The concept of an intrinsic metal-ligand bond energy is defined and used to consider the strengths of bonds in coordinatively saturated metal-ligand compounds. Initial studies that address the metal-ligand bond energies in such species more directly include measurements of the sequential bond energies of M(CO)X + (x = 1–6), M(H20)x + (x = 1–4) and M(CH4)X + (x = 1–4). Variations in these bond energies as the number of ligands increases are interpreted by considering how the valence electrons on the metal reorganize to accommodate the ligand shell. Finally, studies of the thermochemistry of proposed reactive intermediates for the reactions of Fe+ and Co+ with small alkanes are discussed with an emphasis on understanding the potential energy surfaces for C-H and C-C bond activation processes.