Electron transfer and bonding in the transition metal diborides the heats of formation of the metal diborides derived from a theoretical bonding model

Abstract

If interatomic electron transfer changes the bond orders from n o 1 , n o 2 , n o 3 …, to 1, n 2, n 3…, and z 1, z 2, z 3…, are the numbers of bonds of each type, the stability of a given compound is accounted for by the number of transfer electrons Δϵ = z 1( n o 1 − n 1) + z 2( n o 2 − n 2) + z 3( n o 3 − n 3)… occupying the lowest energy bands arising from the interaction of various atomic states. Δϵ is proportional to the heat of formation ΔH f . ΔH f = ΔϵE b where E b is the average energy for all valence bands. This approach is used to correlate the heats of formation of the transition metal diborides with Δϵ values obtained from electron population analyses. The consistency of the results lends support to the electron transfer bonding model and to the hypothesis that boron atoms in a metal diboride form a graphite-type layer structure after increasing their valence with the transfer electrons to a value of four. The delocalization of the transfer electrons between the π orbitals of boron and the empty d orbitals of the metal ensures electroneutrality and the stability of the metal diboride.