Phosphine loss from bis(phosphine)rhodium(I) eta(2)-(C,O)-diphenylketene complexes leading to eta(4)-(C-4) coordination and fluxionality of the ketene

Significant structural changes accompany phosphine loss from bis(phosphine) complexes trans-ClRh[eta(2)-(C,O)-Ph2C=C=O](PR3)(2) [4, where PR3 = PMe(t- Bu)(2), P(i-Pr)(3), or PCy3], giving ClRh[eta(4)-(C-4)-Ph2C=C=O](PR3) (5). As shows by the X-ray crystal structure of 5a [PR3 = PMe(t-Bu)(2)] and vari able-temperature NMR studies, the metal moves from the ketene C,O bond to t he C,C bond and also coordinates weakly to two carbons of a ketene phenyl s ubstituent, resulting in fluxional behavior unique among complexes of aryl- or vinyl-substituted ketenes.