FACTORS AFFECTING SIGMA-BOND ACTIVATION IN SIMPLE SYSTEMS - MEASUREMENT OF EXPERIMENTAL BINDING-ENERGIES OF FE-2)(1-6) CLUSTERS((H)

Dissociation energies and entropies for H-2 loss from Fe+(H-2)(n) clus ters (n = 1-6) have been determined via temperature-dependent equilibr ium measurements. D-0 = -Delta H-0(o) = 10.8 +/- 0.6, 15.7 +/- 0.7, 7. 5 + 0.4, 8.6 +/- 0.4, 2,2 +/- 0.3, and 2.3 +/- 0.3 kcal/mol for n = 1- 6, respectively. The trends in these binding energies are similar to t hose observed for Co+(H-2)(n) and V+(H-2)(n) with some important excep tions. The small value of the first binding energy relative to the sec ond provides strong evidence for curve crossing from ground state Fe(D-6, 3d(6)4S(1)) to the Fe+ (F-4, 3d(7)) surface as the first H-2 lig and approaches. Comparison with recent theoretical calculations has le d us to discard steric hindrance as a primary factor in explaining the decrease in binding energy as ligation number increases in favor of m etal specific covalent interactions. The most important of these are m etal s-d hybridization, sigma donation from H-2 to M(+), and d pi back donation from M(+) to the sigma orbital on H-2.