Bond energies for cationic bare metal hydrides of the first transition series: a challenge to density functional theory

Abstract

Density functional methods based on the Local Density Approximation (LDA) and its nonlocal extensions (LDA/NL) are used to calculate the bond energies, as well as the bond lengths and vibrational frequencies of the high spin, open shell first-row transition metal hydride cations MH⁺. The D₂₉₈(M⁺—H) LDA/NL bond energies are in good agreement with experiment for the early transition metals with errors within 5 kcal/mol. However, the error increases to 6–l3 kcal/mol for the late metal hydrides. An analysis based on atomic properties such as 4s ionization potentials and 4s to 3d promotion energies revealed that the large error in bonding energies among the late transition metals can be attributed to an overestimation of the exchange energy in the DFT schemes. It is shown that a simple remedy, based on a thermodynamic cycle, can improve the agreement between experimental and theoretical bond energies. However, simple cationic bare metal complexes such as MH⁺ remains a challenge to approximate DFT.