Coordination complexes of aryl- and alkylphosphonium cyclopentadienylides (cyclopentadienylidene ylides), C5R4PR′R′′R′′′ (R = H, alkyl, aryl; R′, R′′, R′′′ = alkyl, aryl)

The coordination chemistry of phosphonium cyclopentadienylides C5R4PRR′R′′R′′′ (R = H, alkyl, aryl; R′, R′′, R′′′ = alkyl, aryl) is reviewed critically. In part, perhaps, because of difficulties in characterizing many of the coordination complexes obtained in the early days, research on this class of potentially very interesting ligands stagnated and virtually no publications dealing with their coordination chemistry have appeared in over two decades. As a result, the reactivities of most of these compounds remain largely unexplored and the effects of ligand substitution on metal complex structures and reactivities have been very little examined. Although significant contributions on this subject have been made, much interesting chemistry remains to be examined and discovered using this class of ligand. If the weight of modern spectroscopic techniques is brought to bear on the some of the structural questions that have appeared in the literature, new insights may be garnered. This review does not deal with analogous groups 15 and 16 ylides which have been reported, such as those of sulphur, arsenic and antimony. Although these ylides are related to the phosphorus ylides, it is beyond the scope of this review to discuss their coordination chemistry.