A Position‐Space View on Chemical Bonding in Metal Diborides with AlB2Type of Crystal Structure

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Abstract
On the basis of QTAIM and ELI‐D partitioning of position space two‐ and three‐center delocalization indices were calculated for fifteenMB2phases with the crystal structure of AlB2type. The bonding picture in main‐group metal diborides is closest related to graphite with dominant covalent B–B bonding, albeit with lower effective bond order. For MgB2an exceptionally large distant electron sharing was found. Transition‐metal diborides display smaller effective bond orders B–B but higher effective bond ordersTM–B andTM–TMthan main‐group metal diborides. The large chemical flexibility of this structure type is caused by counterbalancing effects of B–B bonding vs.M–B andM–Mbonding. Different three‐center fluctuation channels of bonds B–B are found for main‐group and transition‐metal diborides, namely B–B–B for the former and B–B–Mfor the latter. With the technique of ELI‐D/QTAIM intersection the increasing importance of B2→4Mbond charge fluctuations along each row of the periodic table can be recovered already at the topological level of analysis.

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