A family of P-iron-substituted phosphinoboranes, Cp(CO) 2 Fe{P(Ar)BMes 2 } (Ar = Ph, Mes, Tipp, Mes*), have been prepared from the reaction of Cp(CO) 2 FeCl and (Li)(Ar)PBMes 2 . All the complexes have been characterized successfully by 1 H, 11 B, and 31 P NMR; IR spectroscopy; and Xray crystallography. In the IR spectra, all the complexes display similar carbonyl stretching frequencies that are remarkably higher than those of closely related phosphide complexes. These observations indicate that a repulsive interaction between the filled d orbital on the iron and the lone pair on the phosphorus is less severe in the studied iron-phosphinoboranes, which is most likely because of the P→B π interaction that occurs in them. The 31 P{ 1 H} NMR chemical shifts of the phosphinoborane phosphorus move upfield with the increasing steric bulk of the Ar groups in the order Ph (−51.4 ppm) < Mes (−68.8 ppm) < Tipp (−84.9 ppm). However, the phosphorus bearing the most sterically demanding Mes* group appears at an unexpectedly downfield value of −44.9 ppm, which is probably reflective of its structural peculiarities. The 1 H NMR spectrum of each complex displays two sets of signals, assignable to inequivalent Mes groups on the boron atom, as a consequence of a hindered rotation around the P−B bond. This high rotational barrier most likely results from the significant double-bond character in the P−B bond. The X-ray diffraction studies have confirmed the iron-phosphinoboranes considered herein to be monomeric species. Each molecule consists of a nearly planar phosphinoborane fragment with a short P−B bond. The Fe−P bond is notably elongated as the Ar group becomes larger, demonstrating its somewhat vulnerable nature with respect to steric congestion. In contrast, the variation in the P−B bond distance is relatively small throughout the series of iron-phosphinoboranes, suggesting that the PB double-bond character is balanced by steric and electronic effects of the substituents around the phosphorus.

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