H-H, C-H, and Related Sigma-Bonded Groups as Ligands

Publisher Summary This chapter discusses H–H, C–H, and sigma-bonded related groups as ligands. Ligands that act by donation of lone pairs (e.g., NH3) or of π-bonding electrons (e.g., C2H4) are common in transition-metal chemistry. Two-electron, three-center bonds seem to form most readily when at least one of the three centers is H. Dihydrogen complexes are the simplest transition-metal species isolobal with H3+, but they have been recognized only in the past three years. Solid-state nuclear magnetic resonance (NMR) spectroscopy has provided further insights into the properties of H2 complexes. The simplest reaction of an H2 complex is dissociation. There is clear evidence in many cases that H2 ligands can undergo rapid exchange with terminal M–H groups. Interactions between a C–H bond, and coordinatively unsaturated metal fragments were first observed in the 1960s by Ibers and by Maitlis. A particularly significant series of agostic complexes is Cp*Co(PR3)-(alkyl). There are a number of cases in which essentially planar methyl groups bridge between two metals. In a recent development, it has been shown that the [HW2(CO)10] anion has a bent W–H–W bridge; previous data had been collected on disordered salts and so were less reliable.