$$C{e^{ + 3}},\,\,L{a^{ + 3}},\,\,P{m^{ + 3}}\,\,$$ and $$Y{b^{ + 3}}\,\,$$ have ionic radial in the increasing order as

Following types of compounds (as $$I, II$$ )



are studied in terms of isomerism in :

A square planar complex is formed by hybridisation of which atomic orbitals ?