Coordination of N2 and Other Small Molecules to the Phosphorus Centre of RPW(CO)5 : A Theoretical Study on the Janus Facets of the Stabilization/Activation Problem.

Bonding of neutral terminal phosphinidene tungsten(0) complexes stabilized by ligands bound to phosphorus (ligand-to-P) have been studied using a large testbed of predominantly N-ligands. Most complexes (1 a-r,v-z and 2 l) exhibit a pyramidal P centre with a formal single ligand-to-P bond order. Three ligand-to-P complexes (1 s-u) exhibit planar sp2 -hybridization of P and P,N bonds featuring double bond character. All derivatives with C-ligands (1 v,x-z) exhibit a ligand-to-P bond strength intermediate between the P-N single and double bonded species. Remarkably, dinuclear complexes containing CN- (1 p,w), N3- (1 r) and N2 (1-3 t) as bridging ligands between two phosphorus centres show intertwined terminal W(CO)5 fragments. In particular, the case of N2 bridging ligand (1-3 t) represents a noteworthy example of activation, displaying a weakened N,N bond and two sets of very strong P,N double bonds, the latter resembling the activation-picture of dinitrogen by transition metal complexes. Bond dissociation energies (BDEs) of ligand-to-P bonds do not show any correlation with a range of bond strength descriptors, but display some meaningful, roughly linear variation with the ligand softness. Based on Haaland's definition of dative bonding, a reasonable linear correlation of BDE with dativity (d1 ) and the dative covalence energy (DCE1 ) was found. A moderate correlation was also obtained with the vertical ionization potential of the ligand (I(L)) as well as with the ligands HOMO energy.