Relation between electronegativity and heat of formation in MX4 halide.

The heats of formation of halides (HfX4 ZrX4, TiX4SiX4CX4), -ΔH°298, are empirically expressed in terms of electronegativities (xC and xA) of cation and anion (halogen ion): -ΔH°298/(4e2/rC)=axA+b/xC-c+dxA+e, where a, b, c, d, and e are empirical constants; factor 4, e, and rC represent the valence number of cation, the charge on the electron, and cation-radius, respectively. The value of 4e2/rC corresponds to electrostatic energy between effective nuclear charge of the M4+ ion (4e) and an electron at a distance from its nucleus equal to its ionic radius rC. Although physical meaning is not clear, this empirical equation is useful in predicting electronegativity and/or ionic radius from the heat of formation, and vice versa. The electronegativity of Hf4+ is revised to be 1.4 in Pauling's scale and the ionic radius of Th4+ in tetrahedral site is estimated to be 0.77Å. The heats of formation of GeX4, SnX4 and PbX4 (X: halogen) are related to both cation radius (rC) and anion radius (rA): -ΔH°298{(-e2/rA)/(-100kcalthmol-1)}1/2/(4e2/rC)=axA+b.From this empirical equation the heat of formation of SnF4, (g) is estimated to be 235 kcalthmol-1.