Question

Determine the coordination number of the central metal ion in (a) \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Br}_{2}\right]\). (b) \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\). (c) \(\left[\mathrm{Ti}\left(\mathrm{H}_{2} \mathrm{O}\right) \mathrm{Cl}_{5}\right]^{2-}\) (d) \(\left[\mathrm{Mn}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{3}\right]^{4-}\)

Short answer

(a) 4, (b) 6, (c) 6, (d) 6.

Step-by-step solution

Understanding the Problem

The coordination number refers to the number of ligand donor atoms bonded to the central metal ion in a coordination complex. Each ligand can have one or more donor atoms.

Analyzing (a) \([\mathrm{Pt}(\mathrm{NH}_3)_{2} \mathrm{Br}_{2}]\)

This complex has two \(\mathrm{NH}_3\) molecules and two bromide ions \((\mathrm{Br}^-)\) acting as ligands. Each ligand provides one donor atom. Thus, the coordination number is 2 (from \(\mathrm{NH}_3\)) + 2 (from \(\mathrm{Br}^-\)) = 4.

Analyzing (b) \([\mathrm{Fe}(\mathrm{CN})_{6}]^{3-}\)

The cyanide ion \((\mathrm{CN}^-)\) is a monodentate ligand, which contributes one donor atom each. With six \(\mathrm{CN}^-\) ligands, the coordination number is 6.

Analyzing (c) \([\mathrm{Ti}(\mathrm{H}_2 \mathrm{O}) \mathrm{Cl}_{5}]^{2-}\)

This complex consists of one water molecule \((\mathrm{H}_2 \mathrm{O})\) and five chloride ions \((\mathrm{Cl}^-)\) as ligands. Both are monodentate, providing one donor atom each. Hence, the coordination number is 1 (from \(\mathrm{H}_2 \mathrm{O}\)) + 5 (from \(\mathrm{Cl}^-)\) = 6.

Analyzing (d) \([\mathrm{Mn}(\mathrm{C}_2 \mathrm{O}_4)_3]^{4-}\)

The oxalate ion \((\mathrm{C}_2 \mathrm{O}_4^{2-})\) is a bidentate ligand, which means each ion contributes two donor atoms. With three oxalate ions, the coordination number is 3 (oxalate ligands) × 2 (donor atoms per ligand) = 6.

Key concepts

Coordination Complex

A coordination complex is a fascinating structure in chemistry where a central metal ion is surrounded by molecules or ions known as ligands. The central metal ion is at the heart of this complex, providing a positive charge that attracts negatively charged or neutral ligands. This creating a stable and unique compound. These complexes play an essential role in various fields, including biochemistry, medicine, and material science.

Components of a coordination complex include:

• Central Metal Ion: Usually a transition metal like iron, nickel, or platinum.
• Ligands: Molecules or ions that donate electron pairs to the metal ion.
• Coordination Sphere: Includes the central metal ion and its attached ligands, often enclosed in square brackets.

Coordination complexes can have different geometrical configurations such as octahedral, tetrahedral, or square planar, depending on the coordination number and types of ligands present.

Ligand Donor Atom

The concept of a ligand donor atom is central to understanding coordination complexes. A ligand is the molecule or ion that binds to the central metal through one or more donor atoms. These donor atoms often contain lone pairs of electrons, which they donate to the central metal ion, thus forming a coordinate covalent bond.

In chemistry, the donor atom is the atom within a ligand that is directly bonded to the metal ion. It is usually a non-metal atom with lone pair electrons, such as nitrogen in ammonia ( NH_3 ) or oxygen in water ( H_2O ). For example, in the complex ext{[Fe(CN)}_6]^{3-} , each cyanide ion ( CN^{−} ) acts as a ligand, with the carbon atom serving as the donor atom.

To summarize, the effectiveness of a ligand as part of a coordination complex largely depends on the nature and number of its donor atoms.

Monodentate and Bidentate Ligands

Ligands can be defined by the number of sites they use to bind to the central metal ion. In a coordination complex, this leads to two primary types: monodentate and bidentate ligands.

• Monodentate Ligands: These ligands possess only one donor atom and bind to the central metal ion at a single point. Classic examples include water ( H_2O ), ammonia ( NH_3 ), and chloride ion ( Cl^− ), where each donates one pair of electrons to the metal.
• Bidentate Ligands: Unlike monodentate ligands, these offer two donor atoms to bind simultaneously. This dual anchoring can create a more robust attachment to the metal ion. A typical instance is the oxalate ion ( C_2O_4^{2-} ), which uses both oxygen atoms to connect with the metal.

Understanding the difference between these two types is crucial, as they influence the stability, shape, and coordination number of the complex.

Central Metal Ion

The central metal ion is the cornerstone of a coordination complex. It is typically a transition metal, possessing vacant d-orbitals that allow it to accept electron pairs from ligands. This ability to coordinate with ligands is crucial in forming the entire structure of the coordination complex.

These metal ions exhibit variable oxidation states and coordination numbers. For example:

• Platinum ( Pt ): In ext{[Pt(NH}_3{)_2 Br}_2] , platinum acts as the central metal ion with a coordination number of 4.
• Iron ( Fe ): In ext{[Fe(CN)}_6]^{3-} , the iron ion is surrounded by six cyanide ligands with a coordination number of 6.

The size, charge, and electron configuration of the central metal ion dictate the overall geometry and reactivity of the coordination complex.