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

Abstract

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.