29658-60-4Relevant articles and documents
Spectral resolution of fluxional organometallics. The observation and FTIR characterization of all-terminal [Rh4(CO)12]
Allian, Ayman D.,Garland, Marc
, p. 1957 - 1965 (2005)
In situ FTIR spectroscopy at 1 cm-1 resolution was conducted on n-hexane solutions of the bridged [Rh4(CO)9-(μ-CO) 3] in the interval T = 268-288 K and PT = 0.1-7.0 MPa using either helium or carbon monoxide as dissolved gas. Analysis of the spectral data sets was conducted using band-target entropy minimization (BTEM), in order to recover the pure component spectra. A new spectral pattern was recovered with terminal vibrations at 2075, 2069.8, 2044.6 and 2042 cm -1. The new spectrum is consistent with an all-terminal [Rh 4(CO)12] species with a C3v anticubeoctahedron structure where 2 different [Rh(CO)3] moieties exist, although the presence of some Td structure can not be entirely excluded. The equilibrium between all-terminal [Rh4(CO)12] and the bridged [Rh4(CO)9(μ-CO)3] was determined in the presence of both helium and CO. The equilibrium constant Keq = [Rh4(CO)12]/[Rh4(CO)9-(μ-CO) 3] at 275 K was ca. 0.011 and the determined equilibrium parameters were ΔrG = 12.63 ± 4.8 kJ mol-1, ΔrH = -21.45 ± 2.3 kJ mol-1 and ΔrS = -114.3 ± 8.35 J mol-1 K-1. The free energy indicates a very small difference between the bridged and terminal geometry, and the lower entropy is consistent with a higher symmetry. This finding helps to address a long-standing issue concerning the existence of various [M4(CO)12] symmetries. In a more general context, the present study illustrates the considerable utility of quantitative infrared spectroscopy (occurring on a fast vibrational timescale) combined with sophisticated deconvolution techniques in order to resolve systems which have been demonstrated to be fluxional on the NMR timescale. The Royal Society of Chemistry 2005.
