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

Equilibrium methods were used to measure ΔHT°, ΔST° and ΔH0° for the sequential clustering of up to 6 H2 ligands to a 3d56S Cr+ core ion. The binding energies are the weakest yet measured for a dn 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” 3d5 Cr+ ion and the H2 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 H2 ligands, and maximize back donation into the σ∗ orbitals of the H2 ligands. A stable planar D3h 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.