Chemistry of the Oxophosphinidene Ligand. 1. Electronic Structure of the Anionic Complexes [MCp{P(O)R*}(CO)2]- (M = Mo, W; R* = 2,4,6-C6H2tBu3) and Their Reactions with H+ and C-Based Electrophiles.

The anionic phosphide-bridged complexes (H-DBU)[M2Cp2(mu-PHR*)(CO)4] (M = Mo, W; R* = 2,4,6-(C6H2Bu3)-Bu-t; Cp = eta(5)-C5H5, DBU = 1,8-diazabicyclo [5.4.0] undec-7-ene) react with molecular oxygen to give the corresponding oxophosphinidene complexes (H-DBU)[MCp{P(O)R*}(CO)(2)] as major products (Mo P = 2.239(1) angstrom for the Mo complex). The latter anionic complexes are protonated by HBF4 center dot OEt2 to give the hydroxyphosphide derivatives [MCp{P(OH)R*}(CO)(2)]. In the presence of excess acid, the molybdenum complex yields the fluorophosphide complex [MoCp(PFR*)(CO)(2)] (Mo P = 2.204(1) angstrom), while the tungsten compound reacts with excess HCl to give an unstable chlorophosphine complex [WCpCl(PHCIR*)(CO)(2)] which is rapidly hydrolyzed to give [WCpCl{PH(OH)R*(CO)(2)], having a complexed aryiphosphinous acid (Mo P = 2.460(2) angstrom). The molybdenum anion reacts with strong C-based electrophiles such as [Me3O]BF4, Et2SO4, C2H3C(O)Cl, and PhC(0)Cl to give the corresponding alkoxyphosphide derivatives [MO)Cp{P(OR)R*}(CO)2] (R = Me, Et, COC2H3, COPh; Mo P = 2.197(2) angstrom for the benzoyl compound), as a result of the attack of the electrophile at the 0 atom of the oxophosphinidene ligand. In contrast, the reactions with milder alkylating reagents such as the alkyl halides Mel, Etl, C3H5Br, and C3H3Br give selectively the corresponding kappa(2)-phosphinite complexes [MoCp{kappa(2)-OP(R)R*}(CO)2] [R = Me, Et, C3H5, C3H3; Mo P = 2.3733(5) angstrom for the allyl compound] as a result of the attack of the electrophile at the P atom of the oxophosphinidene ligand. According to density functional theory (DFT) calculations, the oxygen atom of the phosphinidene ligand bears the highest negative charge in the molybdenum anion, while the highest occupied molecular orbital (HOMO) of this complex has substantial Mo-P pi bonding character. Thus, it is concluded that the phosphinite complexes are formed under conditions of orbital control, while charge-controlled reactions tend to give alkoxyphosphide derivatives.