Electron Counting in Octahedral Clusters
The species Mo6Cl142- feature Mo(II) (d4) centers. Six Mo(II) centers gives rise to a total of 24 valence electrons, or 2e/Mo-Mo vector. More electron-deficient derivatives such as Ta6Cl184- have fewer d-electrons. For example, the naked cluster Ta6+14, the core of Ta6Cl184- would have 5(6) - 14 = 16 valence electrons. Fewer d-electrons result in weakened M-M bonding and the extended Ta---Ta distances accommodate doubly bridging halides.
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