CBSE Class 12 · 2004 · All India Compartment; also Delhi 2003 · d and f Block ElementsShort Answer

(a) Stable complexes form because the central metal ion has:

1. a high charge-to-size ratio (small, highly charged ion attracts ligands strongly), and
2. vacant (n−1)d orbitals of suitable energy to accept the lone pairs donated by ligands.

(b) Zn${}^{2+}$: [Ar]3d${}^{10}$ — completely filled d-subshell, no d–d transition possible, so zinc salts are white.
Cu${}^{2+}$: [Ar]3d${}^9$ — the incomplete d-shell permits d–d transitions in visible light, so copper(II) salts are coloured.