Equilibrium methods were used to measure Delta G(T) degrees as a function of temperature for the sequential clustering of up to six H-2 ligands to the Mn+(3d(5)4s(1 7)S) and Zn+(3d(10)4s(1 2)S) core ions. The resulting binding energies are by far the lowest in the first-row transition metal series, ranging from 3.75 kcal/mol for Zn+(H-2) and 1.9 for Mn+(H-2) down to 1-1.5 kcal/mol for the highly ligated clusters. Density functional theory calculations using the B3-LYP parametrization were performed to provide information on structures, vibrational frequencies, and orbital populations, which, together with the experimental data, give insight into the types of bonding that are present. It was found that the repulsion between the singly occupied 4s orbital and the H-2 sigma orbital accounts for the low binding energies and that the 4p orbitals play a key role in reducing this repulsion.