HETEROLYTIC SPLITTING OF HH, SiH, AND OTHER σ BONDS ON ELECTROPHILIC METAL CENTERS

Publisher Summary This chapter discusses heterolytic splitting of H-H, Si-H, and other σ bonds on electrophilic metal centers. Extensive computational analyses of the structure, bonding, and reactions of coordinated dihydrogen are carried out because of the innate ‘‘simplicity’’ of the H2 ligand. However, this is quite deceptive because M-H2 systems have proven to exhibit astonishingly complex structure, bonding, and dynamics, including quantum mechanical behavior. In principle, any X-Y σ bond can coordinate to a metal center, providing that steric and electronic factors are favorable—for example, substituents at X and Y do not block the metal's access. Heterolytic splitting of X-H bonds via proton transfer to a basic site on a cis ligand or to an external base is a crucial step in both industrial and biological processes. Calculations show that for highly electrophilic M the reduction in back donation is almost completely offset by increased electron donation from H2 to the electron-poor M. One of the old, most significant, and widespread reactions of H2 on metal centers is heterolytic cleavage, which involves essentially breaking the H-H bond into H+ and H– fragments.