Cr +(H 2) n clusters: Asymmetric bonding from a symmetric ion

Abstract

Equilibrium methods were used to measure ΔH T °, ΔS T ° and ΔH 0° for the sequential clustering of up to 6 H 2 ligands to a 3d 5 6S Cr + core ion. The binding energies are the weakest yet measured for a d n ion, averaging 4.5 kcal mol −1 and ranging from ∼9 kcal mol −1 for n = 2 down to ∼1 kcal mol −1 for n = 6. High level density functional theory calculations were performed to provide structural information and to help analyse and interpret the data. One definite result is that detailed covalent interactions between the “spherical” 3d 5 Cr + ion and the H 2 ligands are required to explain the data. Both the vacant 4s orbital and, to a lesser extent, the high energy 4p orbitals are involved in hybridization of the 3d orbitals in order to minimize dσ repulsion, maximize σ donation by the H 2 ligands, and maximize back donation into the σ ∗ orbitals of the H 2 ligands. A stable planar D 3h ion appears for n = 3 and again as the core of a trigonal bipyramid for n = 5. Comparisons with results from other first row transition metal ions are made.