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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 fifteen MB2 phases with the crystal structure of AlB2 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 MgB2 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)4M bond charge fluctuations along each row of the periodic table can be recovered already at the topological level of analysis.