Terminal vs. bridging coordination of CO and NO ligands after decarbonylation of [W2Cp2(μ-PR2)(CO)3(NO)] complexes (R = Ph, Cy). An experimental and computational study.

Compounds [M2Cp2(μ-PPh2)(CO)3(NO)] (M = Mo, W) were prepared by reacting the corresponding radicals [M2Cp2(μ-PPh2)(CO)4] with NO, and displayed a terminal, linear NO ligand arranged cis to the P-donor ligand (Mo-Mo = 3.1400(7) Å). The related PCy2-bridged complex [W2Cp2(μ-PCy2)(CO)3(NO)] was prepared in a one-pot, three step procedure first involving deprotonation of the hydride complex [W2Cp2(μ-H)(μ-PCy2)(CO)4] with K[BH(sec-Bu)3], then oxidation of the resulting salt K[W2Cp2(μ-PCy2)(CO)4] with [FeCp2]BF4 at 243 K, and eventually by reacting the so-formed radical [W2Cp2(μ-PCy2)(CO)4] with NO. Photochemical decarbonylation of the Mo2 complex gave intractable mixtures of products. In contrast, photolysis of the ditungsten complexes yielded the corresponding dicarbonyls [W2Cp2(μ-PR2)(μ-κ1:η2-CO)(CO)(NO)] (R = Ph, Cy) as major products, which were characterized spectroscopically. The latter reacted readily with P(OMe)3 to give the corresponding derivatives [W2Cp2(μ-PR2)(CO)2(NO){P(OMe)3}], displaying a cisoid conformation of the P-donor ligands (P-W-P = 83.7(1)° when R = Cy). Density functional theory calculations on [W2Cp2(μ-PCy2)(μ-κ1:η2-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-κ1:κ1-NO ligand (W-W = 2.677 Å) but their interconversion has a large kinetic barrier. It was concluded that formation of the κ1:η2-CO-bridged isomers in the photolytic experiment is favoured by the cisoid disposition of NO and PR2 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-κ1:η2 coordination mode, since it can establish stronger end-on and side-on interactions with the dimetal centre.