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

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.