The coordination number of \(\mathrm{Ni}^{2+}\) is 4 . \(\mathrm{NiCl}_{2}+\mathrm{KCN}\) (excess) \(\mathrm{A}\) (Cyano complex) \(\mathrm{NiCl}_{2}+\) conc. \(\mathrm{HCl}\) (excess) \(\longrightarrow \mathrm{B}\) (chloro complex) The IUPAC name of \(\mathrm{A}\) and \(\mathrm{B}\) are: (a) Potassium tetracyanonickelate (2), potassium tetrachloronickelate ( 2 ) (b) Tetracyanopotassiumnickelate (2), tetrachloropotassiumnickelate (2) (c) Tetracyanonickel (2), tetrachloronickel ( 2 ) (d) Potassium tetracyanonickel (2), potassium tetrachloronickel (2)

In coordination chemistry, a cyano complex is a compound where the central metal atom or ion is bounded to cyano groups (CN−) as ligands. Cyano ligands are known for being strongly attached to metal ions and are called monodentate since they can attach through only one point, which is the carbon atom. For nickel, with an oxidation state of +2 and a coordination number of 4, a reaction with potassium cyanide (KCN) leads to the formation of a complex where four cyanide ions displace the original ligands (such as chloride ions), resulting in the formation of [Ni(CN)₄]²⁻.

According to IUPAC naming conventions, the complex's name will reflect this structural change. Since there are four cyano groups, the prefix 'tetra-' is used, followed by 'cyano' to indicate the ligand type, and finally 'nickelate' to indicate the central metal with its oxidation state in Roman numerals (II for +2). Hence, the cyano complex in the exercise is correctly named 'potassium tetracyanonickelate(II)', showing both the count and type of ligands as well as the charge-carrying counterion, which is potassium in this case.

A chloro complex forms when a metal ion interacts with chloride ions (Cl−) serving as ligands. Chloride ions are also monodentate ligands, similar to cyano groups, but differ significantly in their electronic and steric properties. In the given example, nickel(II) chloride reacts with an excess of hydrochloric acid, leading to the replacement of weaker ligands by chloride ions due to the high chloride ion concentration, yielding [NiCl₄]²⁻.

The appropriate IUPAC name for such a compound incorporates the number of chloride ligands ('tetra-'), the type of ligands ('chloro'), and the metal's oxidation state. For a negative complex ion like this, the counterion must also be taken into account. Hence, for the chloro complex formed under these conditions, the correct IUPAC name is 'potassium tetrachloronickelate(II)'. The name communicates the number of chloride ligands, their nature, the metal at the core of the complex, and its oxidation state, alongside the potassium ions that balance the charge.

Ligand replacement, or ligand substitution, is a chemical reaction where ligands attached to a central metal ion are replaced by different ligands. This process significantly alters the chemical and physical properties of the resulting coordination compound. Ligands can vary in their binding strength, and in the provided exercise, we see the ligand replacement in action twice: once when cyanide ions replace chloride ions to form a cyano complex, and again when chloride ions in excess replace the original ligands in the nickel complex.

This ligand exchange can be influenced by several factors, such as the concentration of the incoming ligand, the nature of the metal centre, and the reaction conditions. The example illustrates key principles of ligand replacement: the initial NiCl₂ can be transformed into a variety of complexes based on the ligands present in the reaction environment. Understanding this concept is critical for predicting the behavior of transition metals in various chemical contexts and for synthesizing new complexes with desired properties.

The oxidation state of a central metal in a coordination compound reflects the number of electrons it has gained or lost relative to its elemental state. It is a critical aspect of the compound's identity and greatly influences its reactivity and bonding characteristics. In coordination chemistry, the oxidation state helps determine the number of ligands that can attach to the metal, known as the coordination number.

In our educational example, the central metal, nickel, exhibits an oxidation state of +2, denoted as Ni²⁺. This indicates that the nickel ion has lost two electrons, which allows it to form stable complexes with a coordination number of 4. Thus, understanding the oxidation state is essential for the correct naming of coordination compounds in IUPAC nomenclature. It becomes part of the name to differentiate between various ionic states of the metal that can lead to distinctly different compounds despite having the same metal and ligands.