ELECTRONIC AND STERIC CONTROL OF THE FORMATION OF C-H-M FROM M-H-M INTERACTIONS IN CARBON-ATOM CAPPED TRANSITION-METAL CARBONYL CLUSTERS - EFFECT OF THE METAL FRAGMENT

The carbon-capped triangular trimetal cluster with the composition FeCo2(CO)9(CHR) exists in solution as an equilibrium mixture of two tautomers: HFeCo2(CO)9(CR) [I] = FeCo2(CO)9(HCR) [II]; R = Me, Ph, 3-PhC6H4, 3-t-BuC6H4, β-naphthyl, and 2,3-Me2C6H3. The equilibrium constants have been measured by NMR and/or IR methods, while the rate constants for the interconversion of I and II have been obtained by line-shape analysis of the NMR data. Variable-temperature IR spectroscopy has been used to define the temperature dependence of the equilibrium ratio for R = Ph. Although the energy difference between tautomers is small, an analysis of the spectroscopic data for both species shows that the bonding of both the hydrocarbyl fragment and the metal carbonyl fragment change substantially, but in a complementary manner, in going from I to II. The data demonstrate that the substituent on the carbyne carbon can perturb the equilibrium via a steric effect; however, the importance of electronic effects on the equilibrium constant is evident in the comparison of the mixed-metal system with other isoelectronic clusters. © 1990, American Chemical Society. All rights reserved.