ALTERNATIVE SYNTHESIS AND FLUXIONAL BEHAVIOR OF TRIANGULO-OS2RH(CO)9(C5H5), AND TRIANGULO-OS2RH(CO)9(C5ME5)

Thermal reaction between Os2(CO)8(µ-C2H4) and (C5R5)Rh(CO)2 provides a convenient alternative synthesis of the triangulo-Os2Rh(CO)9(C5R5) complexes (R = H (2a), Me (2b)). In solution the molecules are fluxional. The mechanism and energetics of the carbonyl exchange processes have been investigated by VT 13C NMR spectroscopy. For 2a a near low temperature limiting spectrum is obtained at -115 °C, which is consistent with the known solid-state structure of the complex. Carbonyl exchange in 2a is initiated between the Os and Rh centers; the associated ΔG* is 8.4 (4) kcal mol-1 at -80 °C. The same process is still fast in 2b even at -100 °C and results in an intermediate averaged spectrum. At higher temperatures global exchange of CO groups over the three metal centers is seen in 2b; ΔG* = 12.7 (2) kcal mol-1 at 22 °C. The difference in fluxionality of 2a and 2b is explained in terms of the greater electron-releasing nature of the C5Me5 moiety. © 1990, American Chemical Society. All rights reserved.