Question

Hexafluoroaluminate(III) ions are known but hexafluoroborate(III) complex ions are unknown. Account for this observation.

Answer 1

Hexafluoroaluminate(III) ions are known but hexafluoroborate(III) complex ions are unknown because of the inability of boron to expand its covalency beyond 4.

• Boron is present in the second period of the periodic table while aluminium is present in the third period of the periodic table.
• Boron unlike aluminium does not posses any empty d-orbitals in the valence shell.
• The orbitals that can be used by boron for bonding includes only one 2s orbital and three 2p orbitals.
• Aluminium can use its 3s, 3p and 3d orbitals for bonding so it can expand its covalency beyond 4 while boron cannot.