The loss of CO from [Rh12(mu12-Sn)(CO)27]4-: synthesis, spectroscopic and structural characterization of the electron-deficient, icosahedral [Rh12(mu12-Sn)(CO)25]4- and [Rh12(mu12-Sn)(CO)26]4- tetra-anions.

Heating (80 degrees C) the electron-precise, Sn-centred, icosahedral cluster [Rh(12)Sn(CO)(27)](4-) under a nitrogen atmosphere affords in sequence the electron-deficient icosahedral [Rh(12)Sn(CO)(26)](4-) and [Rh(12)Sn(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, [Rh(12)Sn(CO)(26)](4-) and [Rh(12)Sn(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 [Rh(12)Sn(CO)(27)](4-), the electron-deficient cluster [Rh(12)Sn(CO)(25)](4-) undergoes reversible electrochemical reductions.