Structural, acidity and chemical properties of some dihydrogen/hydride complexes of Group 8 metals with cyclopentadienyls and related ligands

The structures of Group 8 metal complexes [LMH2(L')(L ")](+) (L = cyclopent adienyls, hydrotris(pyrazolyl)borate (Tp)) and [LMH2(L')(L ")](2+) (L = 1,4 ,7-triazacyclononane derivatives ((R)Cn), 1,3-(Ph2PCN2)(2)C5H3N (PMP)) are compared. While complexes [(C5R5)MH2(L')(L ")]+ (M = Fe, Ru, Os) at room te mperature can exist in either the dihydride form, or the dihydrogen form., or a mixture of both, the analogous Tp, (R)Cn, and PMP complexes are all in the dihydrogen form. Equilibrium studies have shown that the relative acid ities of these hydride complexes are strongly affected by the auxiliary lig ands, metals, and possibly also the H-H interaction. [TpM(H-2)(PR3)(2)](+) (M = Ru, Os) were found to be slightly more acidic than the analogous [CpMH 2(PR3)(2)](+) complexes, and the dicationic (R)Cn dihydrogen complexes [(R) CnM(H-2)(L')(L ")](2+) are much more acidic than the corresponding Cp and T p analogs. Significant acidity enhancement upon substitution of PPh3 for CO is observed for related dihydrogen complexes such as [RuCl(H-2)(L)(PMP)](), [TpRu(H-2)(L)(PPh3)](+), and [(H)Cn(H-2)(L)(PPh3)](2+) (L = CO, PPh3). A t the end of the article, several reactions which may involve heterolytic c leavage of the dihydrogen ligand and proton transfer from coordinated dihyd rogen to olefin ligands are described. (C) 1999 Elsevier Science S.A. All r ights reserved.