Spectral resolution of fluxional organometallics: Further observations and experimental results for Rh4(σ-CO)9(μ-CO)3

Abstract

In-situ FTIR spectroscopy at 1 cm resolution was conducted on n-hexane solutions of Rh4(σ-CO)9(μ-CO)3 on the interval T = 268-288 K and PT = 0.1-7.0 MPa using either helium or CO as dissolved gas. A new spectral pattern was recovered with terminal vibrations at 2075, 2069.8, 2044.6 and 2042/cm. The new spectrum indicated the presence of either one all terminal structure Rh4(σ-CO)12, or the co-existence of two highly symmetric terminal geometries with similar free energies. The equilibrium between Rh4(σ-CO)12 and Rh4(σ-CO)9(μ-CO)3 was determined in the presence of both helium and CO. The equilibrium constant Keq = [Rh4(σ-CO)12]/[Rh4 (σ-CO)9(μ-CO)3 at 275 K was circa 0.011. The free energy indicated a very small difference between the bridged and terminal geometry, and the lower entropy was consistent with a higher symmetry. This study illustrated the considerable utility of qualitative IR spectroscopy (occurring on a fast vibrational timescale) combined with sophisticated deconvolution techniques to resolve systems which were fluxional on the NMR time scale. This is an abstract of a paper presented at the 229th ACS National Meeting (San Diego, CA 3/13-17/2005).