INSERTION OF SC+ INTO H-2 - THE FIRST EXAMPLE OF CLUSTER-MEDIATED SIGMA-BOND ACTIVATION BY A TRANSITION-METAL CENTER

Dissociation energies and entropies for H-2 loss from Sc+(H-2)(n) clusters (n = 1-5) have been determined via temperature-dependent equilibrium measurements. The measured dissociation energies (D-0 = - Delta H-0(o)) are 6.4 +/- 0.5, 5.4 +/- 0.3, and 5.0 +/- 0.4 kcal/mol for n = 2-4, respectively, and similar to 4.5 kcal/mol for n = 5. For n - 1, two isomeric forms are identified: the electrostatic complex Sc+H2 with D-0 = 2.1 +/- 0.5 kcal/mol and the inserted complex HSc+-H with D-0 - 5.5 +/- 0.3 kcal/mol. These thermodynamic data are compared with other results. Both experimental and theoretical analyses of the data indicate the Sc+ ion is inserted into the first H-2 ligand, although the rate (k = 3-13 X 10(-17) cm(3)/s) is very slow. This rate constant has a negative temperature dependence which is incompatible with a simple insertion energy barrier. A reaction mechanism involving the uninserted Sc+(H-2)(n) clusters is proposed to explain the rate data and is supported by selected ab initio calculations. Optimized structures and vibrational frequencies (calculated at the MP2 level) are also given for the subsequent clusters (n = 2-4) along with the calculated heats of formation. A solvation shell of 6 is observed for Sc+ (two H-atom and four H-2 ligands) consistent with other first-row metals.