The ligand polyhedral model approach to the mechanism of complete carbonyl exchange in [Rh4(CO)12] and [Rh6(CO)16]

Abstract

The Ligand Polyhedral Model (LPM) is applied to the carbonyl scrambling observed on the NMR timescale for the two cluster carbonyls [Rh₄(CO)₁₂] and [Rh₆(CO)₁₆]. For [Rh₄(CO)₁₂] the NMR data is completely consistent with the libration of the Rh₄ central core about a C₃ – rotational axis within the icosahedral shell of twelve CO ligands leading to a single-step concerted exchange process. Rotation of the Rh₄ unit is also found to lead to the formation of a new all terminally bonded isomer which retains the icosahedral shell of CO ligands. Importantly, this new isomer does not have the [Ir₄(CO)₁₂] structure which is known to possess an cube-octahedral ligand shell. With [Rh₆(CO)₁₆] and its substituted derivative [Rh₆(CO)₁₅L] exchange is considered to take place via a polyhedral inter-conversion in which the external tetra-capped truncated tetrahedral shell of carbonyl ligands observed in the ground-state undergoes rearrangement first to the 2 : 6 : 6′ : 2 complementary geometry and then returns to the ground-state structure. This leads to a single-step fully concerted CO exchange. The LPM approach also provides an understanding of the variation of activation energies observed in going from [Rh₄(CO)₁₂] to [Rh₆(CO)₁₆] and of the driving force to the fluxional process.