Kinetic study of the reaction of [Rh 6 (CO) 16 ] with NO 2 − : insertion of the nitrogen atom into a Rh 6 cluster core

The cluster [Rh(6)(CO)(16)] reacts with (PPN)NO(2) [PPN = N(PPh(3))(2)] in dichloromethane to form the known cluster [Rh(6)N(CO)(15)](-) in which the nitrogen atom is encapsulated within a trigonal prismatic Rh(6) cluster. A kinetic study shows that the reactions proceed in six kinetically distinguishable steps followed by another four steps that are kinetically unidentifiable but that can nevertheless be identified speculatively on the basis of analogies to other reactions of carbonyl clusters. Stopped-flow studies show that the initial step involves bimolecular nucleophilic attack on a carbon atom of a CO ligand by an oxygen atom in the NO(2)(-) ion. This is followed by CO loss (via a [CO]-dependent and a [CO]-independent path) with formation of [Rh(6)(CO)(14)(OC[combining low line]ON[combining low line]O)](-). Activation parameters have been obtained for all three steps. [Rh(6)(CO)(14)(OC[combining low line]ON[combining low line]O)](-) has been characterized spectroscopically and has the C and N atoms coordinated to one Rh atom. Reaction of this cluster with CO leads to the fairly rapid establishment of an equilibrium with the adduct [Rh(6)(CO)(n)(OC[combining low line]ON[combining low line]O)](-), where n is most probably 15. Both participants in this equilibrium react more slowly with loss of CO(2), and these reactions show negligible dependence on temperature and are therefore largely entropy controlled. The overall reaction is very solvent dependent and numerous side reactions can be identified if the conditions of reaction are not closely controlled.