Q.With the help of crystal field energy-level diagram explain why the complex [Cr(en)3]3+ is coloured ?

(a) $Ti^{3+}$ ($3d^1$) shows a d-d transition and is coloured, while $Sc^{3+}$ ($3d^0$) has no d electrons and is colourless; chrome plating electrodeposits a protective/decorative chromium layer. (b) Werner's theory rests on primary and secondary valencies, the latter being fixed in number and direction, giving the complex its geometry.

(a)(i) Colour of $[Ti(H_2O)_6]^{3+}$ vs $[Sc(H_2O)_6]^{3+}$:

In $[Ti(H_2O)_6]^{3+}$, titanium is present as $Ti^{3+}$, with configuration $[Ar]3d^1$ - it has one unpaired electron in the d subshell. The six surrounding water ligands split the five degenerate d-orbitals into two sets of different energy (crystal field splitting, $t_{2g}$ lower, $e_g$ higher, in an octahedral field). The single d-electron can absorb a photon of visible light of just the right energy to jump from the lower ($t_{2g}$) to the higher ($e_g$) set - this is a d-d transition. Since some wavelengths of white light are absorbed for this transition, the complementary wavelengths are transmitted/reflected, and the complex appears coloured (violet).

In $[Sc(H_2O)_6]^{3+}$, scandium is present as $Sc^{3+}$, with configuration $[Ar]3d^0$ - there are no d-electrons at all. With an empty d subshell, no d-d electronic transition is possible on absorbing visible light, so the complex is colourless.

(a)(ii) Chrome plating: see the note below - chrome plating is the electrolytic deposition of a thin, hard, corrosion-resistant, decorative layer of metallic chromium onto a base-metal object (usually already given a copper and nickel undercoat for smoothness and adhesion). The object is made the cathode in a bath of chromic acid ($CrO_3$) in dilute sulphuric acid, with an insoluble lead-alloy anode; passing current deposits chromium onto the article, giving it wear resistance, corrosion resistance and a bright finish (used on automobile parts, taps, bathroom fittings, etc.).

(b) Postulates of Werner's theory of coordination compounds:

1. Metals in coordination compounds exhibit two types of valency: a primary (ionisable) valency, satisfied by negative ions and equal to the oxidation state of the metal, and a secondary (non-ionisable) valency, satisfied by negative ions or neutral molecules (ligands) and equal to the coordination number of the metal. …