Dihydrogen, dihydride and in between: NMR and structural properties of iron group complexes

Tabulating the structures and characteristic NMR properties of 17 iron complexes, 98 ruthenium complexes and 70 osmium complexes that contain dihydrogen or compressed dihydride ligands reveals a variety of trends. The H-H bond lengths increase from similar Fe(II) to RU(II) to Os(II) complexes. Iron(II) displays a narrow range of H-H distances for stable complexes. Electronegative atoms Cl and 0, when attached on the metal trans to the dihydrogen ligand, result in elongation of the H-H bond relative to more electropositive atoms H, C, P and N. The family of cyclopentadienyl ligands also causes this elongating effect. The dihydrogen ligands with short H-H distances and weak interactions with the metal, especially on iron and ruthenium are in the fast spinning regime. One exception is the biporphyrin complex of ruthenium with the side-on bridging H-2 ligand which has an H-H distance of 118 pm but is in the fast spinning regime. There are some ruthenium complexes with H-H distances greater than 110 pm that are in the slow motion regime and several complexes of osmium with H-H distances greater than 130 pm that are in this regime. The large J(HH) due to quantum mechanical exchange coupling are observable for some of these osmium complexes with H-H distances in the range of 140-160 pm. The dihydrogen ligands in many complexes appear to have librational motions or other motions that place them in the intermediate motion regime. New equations to correlate J(HD) with H-H distances for ruthenium dihydrogen complexes and for osmium dihydrogen complexes are introduced here. (C) 2008 Elsevier B.V. All rights reserved.