The recognition of the fundamental contributions by G. A. Olah on the elucidation of the structure of nonclassical carbocations, in the form of the award of the Nobel prize for chemistry, has recently emphasized the importance of electron-deficient bonds in the understanding of chemical bonding in organic chemistry. In the field of coordination chemistry, the formulation of electron-deficient bonds has been used for some time to describe nonclassical interactions between atoms. Traditional ligands in coordination chemistry such as amines and phosphanes bond to metal centers through their lone pair of electrons. Synergistic bonding effects dominate in the coordination of π-bonded ligands such as alkenes. In the mid-1980s the discovery of dihydrogen complexes having side-on coordination of H2 gave fresh impetus to transition metal chemistry as well as to the understanding of the interaction of σ-coordinating ligands with transition metals. In the meantime, transiton metal complexes can be obtained with a variety of σ-coordinated X-H fragments, and their mode of bonding can be understood by a common and quite general model. The chemistry of σ-bound silane ligands is particularly varied and well-investigated. These silane ligands enable the investigation of a large range of σ-coordinated metal complex fragments up to complete oxidative addition with cleavage of the SiH bond and formation of silyl(hydrido) complexes, which has thus also widened our general understanding of the bonding of other σ-bound ligands. Whilst there is a large range of isolable and stable H2 and SiR4 complexes available, there are no such alkane analogues known at present. Only when the CH bond is part of a ligand that is already directly bonded to the transition metal center will the resulting chelate effect stabilize this agostic C-H-M interaction. The complexation of SiH4, the simplest heavier homologue of CH4, was achieved recently. This is a further step towards the understanding of the factors which govern σ-complexation of ligands at transition metal centers.
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