CARBONYL LIGAND SUBSTITUTION AT RHODIUM IN THE HETEROMETALLIC CLUSTER[PPN](2)[RE7C(CO)(21)RH(CO)(2)] - A REACTIVITY COMPARISON WITH MONONUCLEAR (ETA(5)-C5H5)RH(CO)(2) AND RELATED-COMPOUNDS

The heterometallic cluster compound [PPN](2)[Re7C(CO)(21)Rh(CO)(2)] re acts with tertiary phosphines and phosphites selectively at the rhodiu m center to form the mono-substituted products [PPN](2)[Re7C(CO)(21)Rh (CO)(L)]. Isolated compounds have been characterized by standard analy tical and spectroscopic methods. Kinetics studies have shown that the reaction proceeds by an overall second-order process, first order in c omplex and first order in nucleophile. The second-order rate constants have been analyzed in terms of electronic and steric effects. The sub stitution rates at the rhodium center in the cluster are compared with those for the analogous mononuclear complexes (eta(5)-C(5)H(4)X)Rh(CO )(2) previously studied by Basolo and co-workers.