The exhibition of the highest coordination number depends on the availability of vacant orbitals in the central atom. Which of the following elements is not likely to act as a central atom in ${\mathrm{MF}}_{6}^{3-}$?

1. B

2. Al

3. Ga

4. In

Hint:  The second-period element does not form six coordinated complex

The lowest atomic number among the given is boron. Boron has atomic number 5 and does not have a vacant d-orbital. The element M is the complex ion ${\mathrm{MF}}_{6}^{3-}$ has a coordination number of six. Boron can show a maximum coordination number of 4. Thus, B can not form a complex of the type ${\mathrm{MF}}_{6}^{3-}$.