The loss of CO from [Rh12(μ12-Sn)(CO)27]4−: Synthesis, spectroscopic and structural characterization of the electron-deficient, icosahedral [Rh12(μ12-Sn)(CO)25]4− and [Rh12(μ12-Sn)(CO)26]4− tetra-anions

Abstract

Heating (80 °C) the electron-precise, Sn-centred, icosahedral cluster [Rh₁₂Sn(CO)₂₇]⁴⁻ under a nitrogen atmosphere affords in sequence the electron-deficient icosahedral [Rh₁₂Sn(CO)₂₆]⁴⁻ and [Rh₁₂Sn(CO)₂₅]⁴⁻ derivatives. The reaction is reversible in solution and the parent compound is quantitatively regenerated upon exposure to carbon monoxide. The reaction course has been unravelled via a combination of Band-target Entropy Minimization (BTEM) IR analysis and X-ray studies. While icosahedral clusters displaying electron counts formally exceeding 13 skeletal electron pairs (SEP) are known, [Rh₁₂Sn(CO)₂₆]⁴⁻ and [Rh₁₂Sn(CO)₂₅]⁴⁻ show for the first time that icosahedral clusters may also be stabilized with a deficiency of SEPs with respect to the requirement based on the cluster-borane analogy. In contrast to the behaviour of the electron-precise cluster [Rh₁₂Sn(CO)₂₇]⁴⁻, the electron-deficient cluster [Rh₁₂Sn(CO)₂₅]⁴⁻ undergoes reversible electrochemical reductions.