Phosphinidene linear coordinated

By writing about complexes containing triple bonds between phosphorus and transition metals, one has to take into account the triple-bond character of phosphinidene complexes which are in a nearly linear coordination mode (type C) in contrast to the usual bent coordination mode D possessing typical double-bond features. Due to the additional r-donation bonding ability of the PR moiety to the metal atom in type C and the observed bond lengths, this type of complexes has to be included into the classes of metal-phosphorus triple bond compounds. Thus, at the end of this review will appear a chapter highlighting the appropriate compounds of type C. 

So far, much of the discussion has focussed on the bonding of phosphinidene units to aluminum, gallium, or indium. However, we are also interested in the coordination of phosphinidenes to transition metals - particularly in cases where the phosphinidene unit coordinates in a terminal fashion. As indicated below, terminal phosphinidene complexes, like imido complexes, can exist in angular or linear forms. 

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