Abstract
The formal 18 electron rhodium(+3) cluster, closo-3, 3-(PPh3)2-3-H-3, 1, 2-RhC2B9H11 (I) and related species are useful catalysts for alkene isomerization, hydrogenation and other reactions under mild conditions. These results appear to be anomalous in view of the stable electron configuration of the rhodium center present in (I). Resolution of this conflict was obtained by the observation that alkyl substitution at both carborane cage carbon atoms of (I) resulted in the isolation and structural characterization of several catalytically active formal 16 electron rhodium(+1) nido-species. The fact that these isolated nido-clusters display all of the kinetic characteristics of closo-(I) in alkene isomerization and hydrogenation reactions suggests that closo-(I) is in equilibrium with a spectroscopically invisible and catalytically active nido-tautomer of similar structure. The proposed general mechanism of alkene isomerization and hydrogenation incorporates this view and is further supported by kinetic studies coupled with isotopic labeling experiments, synthesis and crystallographic results.
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