Subvalent Group 14 metal compounds. Part 10. Syntheses and structures of some bis(metallo)plumbylenes [Pb{Mo(R)(CO)3}2(OC4H8)][R =η-C5H5, η-C5H3(SiMe3)2-1,3, or η-C5Me5]; X-ray crystal structures of [Pb{Mo(η-C5Me5)(CO)3}2(OC4H8)] and [{Pb[Mo(η-C5Me5)(CO)3][Mo(η-C5Me5)(CO)2(µ-CO)]}2]

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Abstract
Treatment of Pb[N(SiMe3)2]2 with [Mo(R)(CO)3H] in tetrahydrofuran (thf) affords the green complexes [Pb{Mo(R)(CO)3}2(thf)][R =η-C5Me5(1a), η-C5H3(SiMe3)2-1,3 (1b), or η-C5H5(1c)]. The brown thf-free analogues [Pb{Mo(R)(CO)3}3](2) are obtained from (1) at 25 °C and 10–2 Torr for R =η-C5H3(SiMe3)2(2b) or η-C5H5(2c), but slightly more forcing conditions (40 °C, 10–2 Torr) are required to convert complex (1a) to [Pb{Mo(η-C5Me5)(CO)3}2](2a). The dimeric green bis(isocarbonyl)-bridged complex {Pb[Mo(η-C5Me5)(CO)3][Mo(η-C5Me5)(CO)2(µ-CO)]}2(3), containing a pair of MoCO–Pb linkages, was obtained from Pb[N(SiMe3)2]2 and [Mo(η-C5Me5)(CO)3H] in a hydrocarbon solvent. The brown toluene solution of complex (3) shows i.r. characteristics similar to those of complexes (2b) or (2c), and is thus believed to contain an isoleptic complex of structure (2a). Addition of triethylphosphine to the brown (2b) yields a green transient believed to be [Pb{Mo[η-C5H3(SiMe3)2](CO)3}2(PEt3)], which is rapidly converted to the brown [Pb{Mo[η-C5H3(SiMe3)2](CO)2(PEt3)}2]. The complexes (2) are thermally labile, decomposing to Pb +[{Mo(R)(CO)3}2], with decreasing facility in the sequence R =η-C5H5 > η-C5Me5 > η-C5H3(SiMe3)2, which is attributed to steric considerations. Complex (3) has three co-ordinate PbII, whereas complexes (1) and (2) are assigned as monomers having three- or two-co-ordinate PbII environments, respectively; all are diamagnetic, whence it is inferred that complexes (2) are V-shaped, having ground-state singlet structures. These structural conclusions are supported by the i.r. and n.m.r. spectra, and by single-crystal X-ray data on the pentamethylcyclopentadienyl complexes (1a) and (3). Complexes (1a) and (3) crystallise in space groups P21/c and P, respectively. Both show a pyramidal three-co-ordinate PbII environment, with Mo–Pb 2.989(2) and 3.019(2)Å[(1a)][or for (3): 2.935(1) and 2.989(1)Å] and Pb–O 2.75(2)Å[(1a)][2.918(8)Å in (3)], and Mo–Pb–Mo 118.33(5)°[(1a)][120.71(2)° in (3)].