Carbonyl compounds of Tc, Re, and Bh: Electronic structure, bonding, and volatility

Abstract

Calculations of molecular properties of M(CO)5 and MH(CO)5, where M = Tc, Re, and Bh, and of the products of their decomposition, M(CO)4 and MH(CO)4, were performed using density functional theory and coupled-cluster methods implemented in the relativistic program suits such as ADF, DIRAC, and ReSpect. The calculated first M-CO bond dissociation energies (FBDEs) of Bh(CO)5 and BhH(CO)5 turned out to be significantly weaker than those of the corresponding Re homologs. The reason for that is the relativistic destabilization and expansion of the 6d AOs, responsible for weaker σ-forth and π-back donations in the Bh compounds. The relativistic FBDEs of M(CO)5 have, therefore, a Λ-shape behavior in the row Tc-Re-Bh, while the non-relativistic values increase toward Bh. Using the results of the molecular calculations and a molecule-slab interaction model, adsorption enthalpies, ΔH ads, of group-7 carbonyl hydrides on quartz and Teflon were estimated for future gas-phase chromatography experiments. It was found that BhH(CO)5 should be almost as volatile as the homologs, although its interaction with the surfaces should be somewhat stronger than that of MH(CO)5 (M = Tc and Re), while the M(CO)4 (M = Tc, Re, and Bh) molecules should be non-volatile. It will, therefore, be difficult to distinguish between the group-7 MH(CO)5 species by measuring their ΔH ads on surfaces of Teflon and quartz with an error bar of ±4 kJ/mol. The trends in properties and ΔH ads of group-7 carbonyl hydrides are similar to those of group-8 carbonyls of Ru, Os, and Hs.