Explain the following :
(i) Low spin octahedral complexes of nickel are not known.
(ii) The `pi` - complexes are known for transition elememts only.
(iii) CO is a stroger ligand than `NH_(3)` for many metals.

Answer

(i) `N(28):[Ar]4s^(2) 3d^(8)` ; `Ni^(2+)(28):[Ar]4s^(@)3d^(8)`
In octahedral comples, Pairing of electrons will not take place even if we have strong field lingand, therefore `Ni` does not from low spin octahedral complex.
(ii) in `pi`-complex ,`sigma` bond is formed by donation of `pi` electrons or lone pair to vacant d-orbital of transition metal and p-bond is formed by back donation of pair of electrons from transition metal to vacant antibonding orbital of alkene or carbon monoxide. These condition are found only in transition are found only in transition metals.
(iii) `Co^(2+)` is oxidised to `Co^(3+)` in presence of strong field ligand because energy needed for oxidation is provided by strong field ligand and `Co^(3+)` is more stable than `Co^(2+)`.