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

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.