Aluminium forms `[AIF_(6)]^(3-)` ion but boron does not form `[BF_(6)]^(3-)` ion. Explain.

Explanation

Boron can show a maximum valency as well as co-ordination number of four. Therefore it cannot form `[BF_(6)]^(3-)` ion where it is to exhibit co-ordination number six. On the other and, aluminium can extend its valency as well as co-ordination number to six due to the presence of vacant 3d orbitals. Therefore, it can form `[AIF_(6)]^(3-)` ion.