Zwitterionic Organometallates

Whereas ion-pairing has been long recognized as playing a key role in organometallic chemistry, the zwitterionic feature of organotransition metal complexes has only recently become an influential factor in the design of cationic metal catalysts. The prospective situation of reversed polarity, with metal fragments formally bearing an integral negative charge, may be envisaged. Besides the well-documented zwitterionic complexes derived from phosphonium ylides, other examples of zwitterionic organometallate complexes, formulated as such on the basis of structural, spectroscopic, or chemical evidence, have been reported in the recent literature and are reviewed here. They are classified according to the nuclearity of the anionic core and the nature of the cationic moiety, which can be a phosphonium, an ammonium, an iminium, a sulfonium, or a cationic organometallic fragment. The stabilization of the non-solvated charge separation is shown to arise from resonance and partial hapticity effects, hydrogen-type bonding, and tight intramolecular ion-pairing made possible by a back-folding of the ligand onto the metal center.