General characteristic chemical properties of the first set of transition elements, titanium to copper — common mistakes

When asked to explain why a complex is coloured, you must mention the splitting of d-orbitals by ligands, the absorption of energy from visible light for d-d electron transition, and that the observed colour is the complement of the light absorbed. Simply stating 'it has a partially filled d-subshell' is insufficient for full marks.

Be prepared to use standard electrode potential (E°) data from the data booklet to predict the feasibility of a redox reaction involving transition metal ions and to explain the relative stability of different oxidation states under standard conditions.

Pay close attention to the formulae of complex ions, including the overall charge, coordination number, and the ligands involved. For example, distinguish between [Cu(H₂O)₆]²⁺ and [CuCl₄]²⁻, noting the change in charge and coordination number.

Memorise the key colour changes for redox reactions involving transition metals. Manganate(VII) ($MnO_4^-$) is deep purple and is reduced to the almost colourless $Mn^{2+}$ ion. Dichromate(VI) ($Cr_2O_7^{2-}$) is orange and is reduced to the green $Cr^{3+}$ ion. These are essential for both qualitative analysis and titration calculations.

No, this is a common misconception. A compound will be colourless if the central metal ion does not have a partially filled d-subshell. For example, compounds containing scandium(III) (Sc³⁺, 3d⁰ configuration), titanium(IV) (Ti⁴⁺, 3d⁰), copper(I) (Cu⁺, 3d¹⁰), and zinc(II) (Zn²⁺, 3d¹⁰) are typically white or colourless because d-d transitions are not possible.

For iron, the first two ionisation energies to remove the 4s electrons are relatively low, forming Fe²⁺. The third ionisation energy to remove a 3d electron is not prohibitively large because the 3d and 4s orbitals are close in energy, allowing stable Fe³⁺ to form. For calcium, after removing the two 4s electrons to form Ca²⁺, the third ionisation energy is extremely high as it would involve removing a core electron from the stable 3p sub-shell.

Not necessarily. While some substitutions do involve a change in coordination number (e.g., octahedral [Cu(H₂O)₆]²⁺ changing to tetrahedral [CuCl₄]²⁻), many do not. For example, when aqueous cobalt(II) ions react with excess ammonia, the six water ligands are replaced by six ammonia ligands, and the coordination number remains six: [Co(H₂O)₆]²⁺ + 6NH₃ → [Co(NH₃)₆]²⁺ + 6H₂O.