Syntheses, properties, and reactivities of nucleophilic phosphinidene complexes [L(n)M=P-R] are reviewed. Emphasis is placed on the electronic tuning of this emerging class of phosphorus reagents, using different ancillary ligands and coordinatively unsaturated transition-metal moieties. The difference in applicability of the established stable 18-electron and transient 16-electron phosphinidenes is addressed.

Novel terminal rhodium-and cobalt-complexed phosphinidenes, Cp*(PR 3 ) and Cp(PPh 3 )CodPAr (8), were obtained by dehydrohalogenation of the primary phosphine complexes Cp*RhCl 2 (PH 2 Ar) (2) and CpCoI 2 (PH 2 Ar) (7) in the presence of a phosphine. X-ray crystal structures are reported for Cp*(PPh 3 )RhdPMes* (3) and Cp(PPh 3 )CodPMes* (8). A comparative reactivity study and a computational survey were performed on the Co-, Rh-, and Ir-containing phosphinidene complexes. All react with organic dihalides to form phosphaalkenes, with the rhodium congener being far more reactive than the iridium and cobalt complexes. Density functional theory calculations give geometrical parameters and 31 P NMR chemical shifts in good agreement with experimental data. The rhodium congeners exhibit the most pronounced charge separation of the RhdP bond, which may explain its higher reactivity. The M-L bond is strong in all Cp(L)MdPH (M ) Co, Rh, Ir) complexes and inhibits reactivity at the metal center. Comparisons with the Zr-containing complex Cp 2 (PH 3 )Zr-PH are made.

Catalyst tuning by changing ligands is a well-established protocol in transition-metal chemistry. N-Heterocyclic carbenes (NHCs) and tertiary phosphines (R(3)P) are the ubiquitous ligand actors. Here we demonstrate that the relative sigma-donor/pi-acceptor ability of the NHC ligand itself can be influenced by a simple substituent-controlled conformational change, thereby directly impacting the reactivity of the transition-metal complex.

The N-heterocyclic carbene (NHC) functionalized phosphinidene complexes [(pCy)(I i Pr2Me2)RuPMes*] (4), [(pCy)(I i Pr2Me2)OsPMes*] (6), and [(Cp*)(I i Pr2Me2)RhPMes*] (7) were generated by a double-dehydrohalogenation−ligation sequence of the corresponding primary phosphine complexes with 3 equiv of NHC. The effect of the NHC ligand on the electronic properties of the phosphinidene complexes [(Ring)(NHC)MPH] (8−16), bearing group 7−9 transition metals and cycloheptatrienyl (Cht+), benzene, and cyclopentadienyl (Cp−) as ancillary ligands, was studied by density functional theory. All ligand−M bond energy strengths increase with the order of the transition metal in the periodic table. The metal−carbene bond (M−NHC) is dominated by σ-interaction from the ligand, but the π-interaction is substantial, contributing up to ∼20% of the total orbital interaction arising from metal to ligand π-back-donation. The charged ligands Cht+ and Cp− have notable effects on the total σ- and π-interactions in the M−NHC bond. On going to the right in the periodic table, the structures show an increase in M−NHC bond energy that concurs with the net charge on the phosphorus atom of the MP bond.