When it comes to metal borides, the electron counting and its correspondence to structure seems weird. For instance, $\ce{MgB2}$ and $\ce{AlB2}$ are isostructural, which is surprising.

Further consider the example of metal hexaborides. $\ce{MB6}$ structures are isostructural for $\ce{M}= \{\ce{Ca}, \ce{La}, \ce{Th}, \ce{Ce}, \ce{K}\}$. How does the bonding in in these metal diborides, hexaborides, and so on occur?

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    $\begingroup$ You need to stop thinking of molecules and start thinking of crystal structures. These compounds (and many more) can all be thought of as a metal structure with the B (and often C, N, and/or O) sitting in particular interstitial sites within the metal structure. The fact that many metal-(B,N,C,O) compounds have a broad range of mutual solid solubility is a hint that the bonding is not what you think. $\endgroup$ – Jon Custer Aug 7 '17 at 14:50

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