CR-2)(N) CLUSTERS - ASYMMETRIC BONDING FROM A SYMMETRICAL ION((H)

Equilibrium methods were used to measure Delta H-T degrees, Delta S-T degrees and Delta H-0 degrees for the sequential clustering of up to 6 H-2 ligands to a 3d(5) S-6 Cr+ core ion. The binding energies are the weakest yet measured for a d(n) ion, averaging 4.5 kcal mol(-1) and r anging from similar to 9 kcal mol(-1) for n = 2 down to similar to 1 k cal mol(-1) for n = 6. High level density functional theory calculatio ns were performed to provide structural information and to help analys e and interpret the data. One definite result is that detailed covalen t interactions between the ''spherical'' 3d(5) Cr+ ion and the Hz liga nds are required to explain the data. Both the vacant 4s orbital and, to a lesser extent, the high energy 4p orbitals are involved in hybrid ization of the 3d orbitals in order to minimize d sigma repulsion, max imize a donation by the H-2 ligands, and maximize back donation into t he sigma orbitals of the Hz ligands. A stable planar D-3h ion appears for n = 3 and again as the core of a trigonal bipyramid for n = 5. Co mparisons with results from other first row transition metal ions are made. (C) 1997 Elsevier Science B.V.