Explain the following given appropriate reason:
i) Nickel does not form low spin octahedral complexes.
ii) The `pi`-complexes are known for the transition metals only.
iii) `Co^(2+)` is easily oxidised to `Co^(3+)` in the presence of a strong ligand.

Explanation

i) Ni(Z) = 28 does not form low spin complex because two inner 3d orbitals are not available to the ligands for the complex formation.
ii) in the formation of `pi`-complexes, d-electrons are involved. These are avialable only from hte atoms of transition metals. Therefore, they form `pi`-complexes. For example, Zeise's salt and Ferrocene.
iii) In the presence of a strong field ligand such as `CN^(-)` ion, crystal field splitting energy is more than the third ionisation enthalapy `(Delta_(1)H_(3))` for hte oxidation of `Co^(2+)` to `Co^(3-)` ion. Therefore, `Co^(2+)` is easily oxidised to `Co^(3+)`.