Transition metal complexes are coloured because electronic transitions occur within split d orbital energy levels. Identify two different factors that affect the colour of complexes of a specific transition metal.

Phenolphthalein indicator is colourless in solutions with a pH less than 8.2 but pink in solutions with a pH greater than 10.0. The molecule dissociates according to the equation:

Explain, in terms of the structures, why the indicator is colourless at \({\text{pH}} < 8.2\) and is pink at \({\text{pH}} > 10.0\).

oxidation state of transition element/number of d electrons/charge on ion;

type/identity/charge density of ligands;

stereochemistry/shape of complex/number of ligands;

molecule colourless because energy absorbed in UV region/not absorbed in visible region;

anion pink because of greater conjugation/more alternating single and double (C=C) bonds;

anion/coloured form/more conjugated form absorbs in visible region/lower energy radiation/green light;

complementary colour seen;

Many scored both marks in (a), although some blanks were seen.

Part (b) was generally well answered, with most showing a good understanding of the material being tested, although a few referred to d-d electron transitions.