Molecular vs. solid-state condensed octahedral transition-metal chalcogenide clusters rule-breakers again

4 Molecular vs. solid-state condensed octahedral transition-metal chalcogenide clusters rule-breakers again 

In each of the molecular-cluster chapters we eventually came to the rule-breakers. We are at that point again. Our example deals with metal clusters of a familiar geometry that are found in the Chevrel phases. Young readers will not appreciate 

Look again at the DOS diagram. If two extra electrons could be added per M09S11 unit, they would fill the lower e symmetry MO of the isolated cluster or fill the band at the Fermi level. The latter would render the compound semiconducting. A way to do this is to add a dopant. Experimentally this is done by insertion of Cu into K2M09S11 by a redox reaction which leads to Q12K1.8M09S11. This is successful as 

Answer. The metal count is 59 (9 x 9 (Co) — 11x2 ([Se]2—) = 59). This is 25 more than for [Mo9Sen]2 Indeed a large gamut of electron counts is possible for these condensed octahedral clusters, either molecular or part of extended solids. They are nice examples of compounds which can display variable electron count with constant shape. This situation is somewhat reminiscent of the transition-metal elements which exhibit a metallic d band which is gradually filled as we go from the left to the right of the periodic table without substantial change in their structure. Also review the discussion of cubic clusters in Section 5.2.5. 

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