LIGAND ADDITIVITY EFFECTS AND PERIODIC TRENDS IN THE STABILITY AND ACIDITY OF OCTAHEDRAL ETA-2-DIHYDROGEN COMPLEXES OF D6 TRANSITION-METAL IONS

An original approach for predicting the properties of eta(2)-dihydrogen complexes M(H2)L5 is described based on additive electrochemical parameters for the ligands and Lever's correlations for transition metals of groups 6-8. A correlation for group 9 is also reported. On the basis of the properties of the approximately 70 known d6 octahedral dihydrogen complexes, it is concluded that there is a narrow range of possible energies of the d(pi)(t2g) electrons where stable bonding of the eta(2)-H-2 ligand is possible at 25-degrees-C. This range, defined in terms of electrochemical potentials E1/2(d5/d6) of corresponding dinitrogen complexes M(N2)L5, appears to depend on the ligand trans to N2 but is independent of the metal and of the charge on the complex. This means that progressively more electron-donating ligand sets are needed to stabilize eta(2)-H-2 (or N2) complexes on going from group 6 metals to group 9 d6 metal ions. Complexes M(N2)L5 with potentials above 2.0 V vs NHE correspond to M(H2)L5 species that lose H2 irreversibly at 25-degrees-C whereas complexes with potentials below 0.5 V correspond to to dihydrides, M(H)2L5. Even if the energetics of the d electrons are correct, intramolecular homolytic splitting of dihydrogen might still occur if the product dihydride is especially stable. The potentials, E1/2(d5/d6), that limit stability are more negative when the eta(2)-H-2 ligand is trans to CO than to a sigma-donar ligand. A ligand additivity model provides a guide to the combinations of ligands which are likely to produce stable dihydrogen complexes. The narrow range of electrochemical potentials for stable eta(2)-H-2 complexes translates into a possible range of pK(a) values of about 0-40 for complexes M(eta(2)-H-2)L5. Very acidic dihydrogen complexes (pK(a) < 0) will be very labile with respect to H-2 loss at 25-degrees-C. Dihydrogen complexes are proposed as intermediates in some previously reported reactions, and the predicted properties of postulated dihydrogen complexes are shown to be consistent with the reactivity observed. The dinitrogen stretching frequency of corresponding complexes M(N2)L5 can also be predicted fairly reliably.