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

Abstract On the basis of QTAIM and ELI‐D partitioning of position space two‐ and three‐center delocalization indices were calculated for fifteen M B 2 phases with the crystal structure of AlB 2 type. 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 MgB 2 an exceptionally large distant electron sharing was found. Transition‐metal diborides display smaller effective bond orders B–B but higher effective bond orders TM –B and TM – TM than main‐group metal diborides. The large chemical flexibility of this structure type is caused by counterbalancing effects of B–B bonding vs. M –B and M – M bonding. 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– M for the latter. With the technique of ELI‐D/QTAIM intersection the increasing importance of B 2 →4 M bond charge fluctuations along each row of the periodic table can be recovered already at the topological level of analysis.