Question

Explain, giving reasons:
i) Transition metals and most of their compounds are paramagnetic.
ii) Transition metals generally form coloured compounds.

Hint:

Unpaired d electrons cause paramagnetism (moment \(\mu = \sqrt{n(n+2)}\) BM); d-d transitions in partly filled d orbitals cause colour. Empty or full d (Sc³⁺, Zn²⁺) are colourless and diamagnetic.

Answer 1

Step 1 (part i): Paramagnetism is caused by the presence of unpaired electrons. Transition metals have partly filled (n-1)d orbitals, so their atoms and most of their ions carry one or more unpaired d electrons and are therefore paramagnetic. The strength of paramagnetism is given by the spin-only magnetic moment \(\mu = \sqrt{n(n+2)}\) BM, where n is the number of unpaired electrons; more unpaired electrons means a larger moment.
Step 2: Species with no unpaired electrons, such as Sc³⁺ (d⁰) and Zn²⁺ (d¹⁰), are diamagnetic, which confirms the link between unpaired d electrons and paramagnetism.
Step 3 (part ii): In the presence of ligands the five d orbitals split into two energy levels (t_2g lower and e_g higher). Because the d subshell is only partly filled, an electron can jump from the lower to the higher d level by absorbing a definite wavelength of visible light, called a d-d transition.
Step 4: The colour we see is the complementary colour of the light absorbed. Ions with empty (d⁰) or completely filled (d¹⁰) d orbitals cannot undergo d-d transitions and are therefore colourless, for example Sc³⁺ and Zn²⁺.