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Issue 37, 2015
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The ligand polyhedral model approach to the mechanism of complete carbonyl exchange in [Rh4(CO)12] and [Rh6(CO)16]

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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.

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

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Article information


Submitted
31 Oct 2014
Accepted
25 Mar 2015
First published
25 Mar 2015

Dalton Trans., 2015,44, 16602-16610
Article type
Paper

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

B. F. G. Johnson, Dalton Trans., 2015, 44, 16602
DOI: 10.1039/C4DT03360D

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