A Position-Space View on Chemical Bonding in Metal Digallides with AlB2 Type of Structure and Related Compounds

The main focus of this work was to investigate substitution effects on the chemical bonding in compounds of AlB 2 -type and related structure types. Delocalization indices within the QTAIM approach and the topological analysis of the ELI functionals were used as tools to describe the bonding situation in digallides and diborides. Digallides of AlB 2 -type were found only within group I and II; for CaGa 2 (meta-stable phase), SrGa 2 , BaGa 2 , YGa 2 and LaGa 2 compounds. Within these compounds, QTAIM analysis showed similar trend as previously found in diborides. That is, along the period in the Periodic Table, metal-triel interactions increase at the expense of in-plane (triel-triel) ab interactions (Tr=triel). However, transition metal diborides adopt the AlB 2 -type up to group VI. To understand this difference, we simulated transition metal (TM) digallides and diborides up to group VI in the AlB 2 -type. Additionally, the puckered variants diborides ReB 2 and OsB 2 were also simulated in the AlB 2 -type. With filling of d shell, there is a delicate balance between increase of TM–Tr and decrease of in-plane (Tr–Tr) ab electron sharing. This balance is maintained as long as interlayer interactions in the c direction (Tr–Tr ) c and (TM–TM ) c are not relatively too high in comparison to in-plane electron sharing. In contrast to TM B 2 of AlB 2 -type, digallides in the same structure type build up strong interlayer interactions for early transition metal elements. Our results showed that within digallides, a relatively strong increase in interlayer electron sharing (Ga–Ga) c and (TM–TM ) c takes place. Such increase occurs already for ScGa 2 and TiGa 2 . On the other hand, diborides show a steady increase in electron sharing of TM –B and (TM–TM ) c , but not of (B–B) c . Therefore, it is reasonable to suggest that diborides will tend to adopt a 3D network composed of boron and transition metal atoms (ReB 2 and RuB 2 types). The additional high (Ga–Ga) c interlayer interactions indicate a tendency for digallides ...