STUDIES ON BOND AND ATOMIC VALENCES .1. CORRELATION BETWEEN BOND VALENCE AND BOND ANGLES IN SB-III CHALCOGEN COMPOUNDS - THE INFLUENCE OF LONE-ELECTRON PAIRS

In the present bond-valence concept the bond-valence parameter r(o) is treated as constant for a given pair of atoms, and it is assumed that the bond valence s(ij) is a function of the corresponding bond length D-ij, and that the atomic valence is an integer equal to the formal o xidation number V-for(i) derived from stoichiometry. However, from a s tatistical analysis of 76 [(SbSn)-S-III] and 14 [(SbSen)-Se-III] polyh edra in experimentally determined structures, it is shown that for Sb- III-X bonds (X = S, Se), r(o) is correlated with <(alpha)over bar (i)> , the average of the X-Sb-X angles between the three shortest Sb-X bon ds. This is interpreted as a consequence of a progressive retraction o f the 5s lone-electron pair from the Sb-III nucleus, which can be cons idered as continuous change of the actual atomic valence V-act(i) of S b from +3 towards +5. A procedure is derived to calculate an effective atomic valence V-eff(i) of Sb-III from the geometry, <(alpha)over bar (i)> and D-ij, of the [Sb(III)X(n)] polyhedra, which approximates V-a ct(i) and is a better description of the actual valence state of Sb-II I than the formal valence V-for(i). Calculated V-eff(Sb?III) are found to vary between +2.88 and +3.80 v.u. for [(SbSn)-S-III] and between 2.98 and +3.88 v.u. for [(SbSen)-Se-III] polyhedra. It is suggested th at a corresponding modification of the present bond-valence concept is also required for other cations with lone-electron pairs.