¹³C and ³¹P NMR studies on geometrical non-rigidity in heterobinuclear complexes [WM(CO)₃(PPh₃)₂(η⁵-C₅H₅)] (M Rh, Cu)

¹³C NMR studies on [W₂(CO)₆Cp₂] (Cp = ?⁵-C₅H₅), including a 2D EXSY study, confirm that simple rotation about the WW bond is responsible for the isomerisation in solution. Variable temperature ³¹P{¹H} and ¹³C{¹H} NMR spectra of [WRh(µ-CO)₂(CO)(PPh₃)₂Cp] establish that a pseudo-rotational process, with ?G 27.9 ± 0.8 kJ mol^-1 at -108 ± 8°C, results in interchange of the inequivalent PPh₃ groups and all the CO ligands. The mechanism of this facile fluxional process is discussed in relation to that of [W₂(CO)₆Cp₂], and an intermediate (or transition state) of the type [WRh(µ-CO)₃(PPh₃)₂Cp] is proposed. Variable temperature ³¹P{¹H} and ¹³C{¹H} NMR spectra of [WCu(CO)₃(PPh₃)₂Cp] in solvents of various polarities establish a related geometrical non-rigidity for this complex, but with ?G ca. 39-41 kJ mol^-1, in the range -57 to -69°C; this result, considered along with the very low electrical conductivity in acetone, suggests that an intramolecular mechanism also operates for this interchange. © 1989.