3.26 SeCl6 2-, TeCl6 2-, and ClF6 – are all octahedral, but SeF6 2- and IF6 – are distorted, with a lone pair on the central atom apparently influencing the shape. Suggest a reason for the difference in shape of these two groups of ions. (See J. Pilmé, E. A. Robinson, R. J. Gillespie, Inorg. Chem ., 2006 , I have found the answer here, but quite not satisfied with answer presented here. Please I need a good explanation
The Correct Answer and Explanation is:
The difference in shape between the two groups of octahedral ions—SeCl6 2-, TeCl6 2-, ClF6 – (which are perfect octahedra) and SeF6 2- and IF6 – (which are distorted)—can be explained by considering the influence of lone pairs on the central atom as well as the electronegativity of the ligands.
- Electron Pair Repulsion: The basic idea behind the geometry of these species is that electron pairs (bonding and nonbonding) around a central atom will try to arrange themselves to minimize repulsions according to VSEPR theory. In ideal octahedral complexes like SeCl6 2-, TeCl6 2-, and ClF6 -, all six positions are occupied by bonding pairs of electrons, and thus the repulsions between these bonding pairs are symmetrically distributed. This results in a perfect octahedral geometry.
- Lone Pairs: In SeF6 2- and IF6 -, however, there is a lone pair of electrons on the central atom (Se or I). Lone pairs occupy more space than bonding pairs because they are localized closer to the nucleus and have higher repulsive force. This leads to distortions in the geometry, as the lone pair pushes away the bonding pairs. This results in a less symmetrical arrangement than the ideal octahedral shape, causing a distortion in bond angles and possibly even in bond lengths.
- Electronegativity of Ligands: The ligands’ electronegativity also plays a significant role. Fluorine, being highly electronegative, pulls electron density away from the central atom more effectively than chlorine. This greater pull on the central atom increases the repulsion between the bonding pairs, which, combined with the lone pair, may further exacerbate the distortion in SeF6 2- and IF6 – compared to the Cl-based complexes.
- Effect of Central Atom: The central atoms in these ions—Se, Te, I—also vary in their size and polarizability, influencing the distribution of electron density. For example, iodine in IF6 – is larger than selenium in SeF6 2-, and this larger atomic size may help to somewhat mitigate lone-pair repulsion, but not enough to prevent the distortion.
Thus, the main difference arises from the presence of the lone pair on the central atom in SeF6 2- and IF6 -, which causes repulsions that distort the octahedral geometry, combined with the effects of ligand electronegativity on the central atom.
