Which compound is formed when excess of \(\mathrm{KCN}\) is added to an aqueous solution of copper sulphate? (a) \(\mathrm{Cu}(\mathrm{CN})_{2}\) (b) \(\mathrm{K}_{2}\left[\mathrm{Cu}(\mathrm{CN})_{6}\right]\) (c) \(\mathrm{K}\left[\mathrm{Cu}(\mathrm{CN})_{2}\right]\) (d) \(\mathrm{K}_{3}\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]\)

Coordination chemistry is the study of compounds that feature central atoms or ions (usually metals) surrounded by molecules or anions called ligands. These ligands are attached to the central ion via coordinate covalent bonds. A pivotal example of a coordination compound is the one formed when an excess of KCN is added to copper sulphate in aqueous solution.

In this scenario, the copper ion acts as the central metal ion, and the cyanide ions from the KCN act as ligands. Coordination compounds are characterized by the number of ligand atoms coordinated to the central metal, known as the coordination number, which in this case is four. The resulting coordination compound exhibits properties different from those of the individual ligands or the metal atoms alone, which is important in various applications such as in catalysis, materials science, and medicine.

Stoichiometry in chemistry involves the calculation of reactants and products in chemical reactions. It is a quantitative measure based on the balanced chemical equation and the conservation of mass and charge.

When isolating the compound formed between KCN and copper sulphate, stoichiometry helps us to determine the correct proportion of reactants needed to form the final product without imbalance. Since copper has a +2 charge and cyanide carries a -1 charge, we can deduce that, for electrical neutrality, two cyanide ions would be needed for every copper ion if forming a simple ionic compound. However, given the excess of KCN and the tendency of copper to form a complex with a coordination number of four, we observe that four cyanide ions coordinate to one copper ion, leading to a complex with an overall -2 charge, which must be balanced by potassium ions.

Ligand bonding refers to the interaction between a ligand and a metal ion at the center of a coordination complex. A ligand can be an anion or a neutral molecule that possesses at least one pair of electrons suitable for donation and form a coordinate covalent bond with the metal ion.

In the reaction described, cyanide ions act as ligands that donate a pair of electrons to the copper ion to form a coordination complex. Cyanide is known as a monodentate ligand, which means it forms a single bond with the metal. In the presence of excess KCN, multiple cyanide ions bond with a single copper ion. Understanding ligand bonding is crucial because it influences the geometry, color, reactivity, and magnetic properties of coordination complexes — critical factors in various chemical applications including environmental, biological, and industrial processes.