Stereochemical nonrigidity of [Rh6(CO)15L] clusters in solution

A series of substituted hexarhodium carbonyl clusters [Rh-6(CO)(15)L] (L = PR3 (R = alkyl, aryl), P(OPh)(3), NCMe, I-) has been studied by variable temperature and two-dimensional, X-{Rh-103}, (X = C-13, P-31) HMQC and C-13 EXSY NMR spectroscopy in solution. At low temperatures, the spectra are consistent with retention of the solid state structure. Different localised exchanges of terminal (COt) and face-bridging (COfb) CO's are found to occur over different atoms of the Rh-6-octahedron at higher temperatures and the different pathways of the exchanges are discussed. When L = PR3 (R = alkyl, aryl), the lowest energy scrambling surprisingly involves exchange of COt and COfb, associated with the substituted rhodium ((S) under bar -type), with concomitant exchange of L between the two terminal sites on the substituted rhodium, followed by other localised stereospecific exchanges involving CO's associated with unsubstituted rhodium atoms ((U) under bar -type). For the other substituted clusters (L = P(OPh)(3), NCMe, I-), only (U) under bar -type exchanges are observed. The kinetics of these exchanges are reported at different temperatures and for the (S) under bar -type exchange mechanism, the rate is found to vary with the nature of PR3.