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

Dissociation energies and entropies for H-2 loss from Sc+(H-2)(n) clus ters (n = 1-5) have been determined via temperature-dependent equilibr ium 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, tw o 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 indic ate the Sc+ ion is inserted into the first H-2 ligand, although the ra te (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 i nsertion energy barrier. A reaction mechanism involving the uninserted Sc+(H-2)(n) clusters is proposed to explain the rate data and is supp orted by selected ab initio calculations. Optimized structures and vib rational frequencies (calculated at the MP2 level) are also given for the subsequent clusters (n = 2-4) along with the calculated heats of f ormation. A solvation shell of 6 is observed for Sc+ (two H-atom and f our H-2 ligands) consistent with other first-row metals.