(a) Complex ion is made of metal centre and ligands,and they are linked together through coordinate covalent bonds.
(b) ligand is Lewis acid,contains lone pair of electrons,while metal ion,generally speaking,is Lewis acid,contains empty orbitals.So the coordination process is,in fact,Lewis acid/base reaction (or electron pair donor/acceptor interaction).
(c) (i）[Ni(H2O)6]2+:pale green; [Ni(NH3)6]2+:blue
(ii) [Co(H2O)6]2+:pink; [CoCl4]2-:deep blue
(d) (i) Pt(NH3)2Cl2:square planar,can have trans-,or cis-isomers; Pt(II)
(ii) pseudo-octahedral geometry (D4h symmetry),two H2O ligands occupy axial positions,and 4 NH3 are in equatorial plane.Cu(II).