A pnictogen of peculiar posture.

In 2001, Bridgeman et al. synthesized a rare species having a T-shaped phosphorus atom centering an organometallic cluster, [{Cp(OC)2Mo}2PMn(CO)4]. Our DFT and extended Hückel electronic structure calculations indicate that the T-shaped geometry results from an unusual Mo-Mn-Mo three-center, two-electron bond that "ties" back the two-center, two-electron Mo-P bonds. Additional stabilization of the planar structure arises from Mo-P pi back-donation in a three-center, four-electron Mo-P-Mo pi bond. We find that this compound best resembles a mu2-phosphinidene system, and we also attempt to reconcile 18- and 8-electron rule considerations with the delocalized bonding in this intriguing cluster.