A NUCLEAR MAGNETIC RESONANCE AND MASS SPECTRAL STUDY OF SOME ALKENE COMPLEXES OF RHODIUM(I) AND IRIDIUM(I): COORDINATED ALKENE ROTATIONAL BARRIERS, APPEARANCE POTENTIALS AND ESTIMATED HEATS OF FORMATION

Abstract

The activation energies (ΔG) for olefin rotation in ethylene complexes of Rh(I) and Ir(I) have been estimated by signal coalescence from their temperature dependent ¹H nmr spectra in solution. The barriers decrease with increase in the electron accepting nature of the associated ligand (η⁵-cyclopentadienyl, pentane-2, 4-dionate, carboxylate and substituted derivatives thereof) in accordance with the expected decrease in the importance of the π-contribution to the metal-olefin bond. Barriers are substantially higher for Ir(I) than for corresponding Rh(I) complexes. The complex temperature dependent spectra of di-μ-chlorotetrakis(4-methylpenta-1, 3-diene dirhodium(I) are interpreted in terms of both intra- and intermolecular processes.

The greater ΔG values found for Ir(I) compared to Rh(I) complexes are reflected in the different mass spectral fragmentation patterns for corresponding members of the two series. Appearance potentials and heats of formation are estimated for [cpRh(C₂ H₄)₂] and [cpRh(CO)₂].