Terminal vs. bridging coordination of CO and NO ligands after decarbonylation of [WCp(μ-PR)(CO)(NO)] complexes (R = Ph, Cy). An experimental and computational study.

Compounds [MCp(μ-PPh)(CO)(NO)] (M = Mo, W) were prepared by reacting the corresponding radicals [MCp(μ-PPh)(CO)] with NO, and displayed a terminal, linear NO ligand arranged cis to the P-donor ligand (Mo-Mo = 3.1400(7) Å). The related PCy-bridged complex [WCp(μ-PCy)(CO)(NO)] was prepared in a one-pot, three step procedure first involving deprotonation of the hydride complex [WCp(μ-H)(μ-PCy)(CO)] with K[BH(sec-Bu)], then oxidation of the resulting salt K[WCp(μ-PCy)(CO)] with [FeCp]BF at 243 K, and eventually by reacting the so-formed radical [WCp(μ-PCy)(CO)] with NO. Photochemical decarbonylation of the Mo complex gave intractable mixtures of products. In contrast, photolysis of the ditungsten complexes yielded the corresponding dicarbonyls [WCp(μ-PR)(μ-κ:η-CO)(CO)(NO)] (R = Ph, Cy) as major products, which were characterized spectroscopically. The latter reacted readily with P(OMe) to give the corresponding derivatives [WCp(μ-PR)(CO)(NO){P(OMe)}], displaying a cisoid conformation of the P-donor ligands (P-W-P = 83.7(1)° when R = Cy). Density functional theory calculations on [WCp(μ-PCy)(μ-κ:η-CO)(CO)(NO)] and several potential isomers revealed that this electron-precise molecule (W-W = 3.121 Å) is almost isoenergetic with an unsaturated isomer having a μ-κ:κ-NO ligand (W-W = 2.677 Å) but their interconversion has a large kinetic barrier. It was concluded that formation of the κ:η-CO-bridged isomers in the photolytic experiment is favoured by the cisoid disposition of NO and PR ligands at the parent tricarbonyls, which precludes the NO ligand from easily rearranging into a bridging position after decarbonylation. The above calculations also revealed that the CO ligand is much better suited than NO for the μ-κ:η coordination mode, since it can establish stronger end-on and side-on interactions with the dimetal centre.