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=cyclopentadienyls, hydrotris(pyrazolyl)borate (Tp)) and [LMH2(L′)(L′′)]2+ (L=1,4,7-triazacyclononane derivatives (RCn), 1,3-(Ph2PCH2)2C5H3N (PMP)) are compared. While complexes [(C5R5)MH2(L′)(L′′)]+ (M=Fe, Ru, Os) at room temperature can exist in either the dihydride form, or the dihydrogen form, or a mixture of both, the analogous Tp, RCn, and PMP complexes are all in the dihydrogen form. Equilibrium studies have shown that the relative acidities of these hydride complexes are strongly affected by the auxiliary ligands, metals, and possibly also the H–H interaction. [TpM(H2)(PR3)2]+ (M=Ru, Os) were found to be slightly more acidic than the analogous [CpMH2(PR3)2]+ complexes, and the dicationic RCn dihydrogen complexes [RCnM(H2)(L′)(L′′)]2+ are much more acidic than the corresponding Cp and Tp analogs. Significant acidity enhancement upon substitution of PPh3 for CO is observed for related dihydrogen complexes such as [RuCl(H2)(L)(PMP)]+, [TpRu(H2)(L)(PPh3)]+, and [HCnRu(H2)(L)(PPh3)]2+ (L=CO, PPh3). At the end of the article, several reactions which may involve heterolytic cleavage of the dihydrogen ligand and proton transfer from coordinated dihydrogen to olefin ligands are described.