Which one of the following elements is unable to form M F 6 3 - ion?

Step-by-step solution

As ‘B’ has no d-orbitals, so it can’t extend its covalency beyond 4. Hence, ‘B’ cannot form the ion like MF 6 3 − , i.e., BF 6 3 − . As all other metals have vacant d-orbitals in their valence shells, so they can extend their covalency beyond 4.