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 the shape of the two groups of ions—SeCl6 2-, TeCl6 2-, ClF6 – (which are all octahedral) and SeF6 2- and IF6 – (which are distorted)—can be explained primarily by the presence of lone pairs on the central atom and the resulting electronic repulsion. Here’s a detailed breakdown of the reasoning:
- Octahedral Geometry and Lone Pairs: In ideal octahedral geometry, a central atom forms six bonds with surrounding atoms. However, if there are lone pairs on the central atom, the arrangement of these bonds can be influenced by the lone pair’s repulsion. Lone pairs are not involved in bonding but still exert a repulsive force on the bonding pairs, often causing distortions in the bond angles and overall shape of the molecule.
- SeCl6 2-, TeCl6 2-, and ClF6 –: These species are all octahedral because the central atoms (Se, Te, and Cl) in these ions do not possess lone pairs. Both Se and Te are in Group 16 and have enough electrons to form six bonds with halogen atoms, which fit perfectly into an octahedral arrangement. ClF6 – has a similar bonding scenario with no lone pair on the central Cl atom, resulting in a symmetrical octahedral shape.
- SeF6 2- and IF6 –: In these ions, the central atoms (Se and I) are larger and more electronegative compared to their halogen counterparts. When a lone pair is present, as in SeF6 2- and IF6 -, the repulsion from this lone pair distorts the ideal octahedral geometry. Lone pairs tend to occupy more space than bonding pairs because they are localized closer to the nucleus, leading to an uneven distribution of electron density. This results in a distortion of the angles between the bonds, making the geometry less regular than the perfect octahedral shape.
- Electron Pair Repulsion: The distortions in SeF6 2- and IF6 – are due to the interaction between the lone pair and the bonding pairs, which causes the angles to deviate from the ideal 90° or 180° found in a regular octahedron. This effect is more pronounced in SeF6 2- and IF6 – because the lone pair on the central atom is larger and more repulsive than the bonding pairs, causing noticeable distortions.
In conclusion, the difference in geometry between the two groups of ions can be attributed to the presence or absence of lone pairs on the central atom. Lone pairs cause distortions by increasing electron-electron repulsion, altering the ideal octahedral shape.
