Metal complexes of planar PR2 ligands: Examining the carbene analogy

Can we classify planar M–PR2 complexes in the same way as we do planar M–CR2 complexes? The short answer is “yes”. This review attempts a longer answer. The analogy between phosphenium ligands, PR2+, and Fischer carbenes is well established, but a broader analogy includes comparison of planar phosphido ligands, PR2−, with Schrock carbenes. Examples of complexes of planar PR2 ligands from across the transition series are categorized as “phosphenium” or “planar phosphido” in character, based on their structures, reactivities, and/or syntheses. Also presented are complexes with structures and behaviours that lie between these two extremes.

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► Planar M–PR2 complexes can be classified in the same way as planar M–CR2 complexes.
► Phosphenium ligands (PR2+) are compared with Fischer carbenes.
► Planar phosphido ligands (PR2−) are compared with Schrock carbenes.
► This useful analogy is complicated by the extra valence e− at P relative to C.
► Structure and reactivity of these complexes may lie between the two extremes.