Question

Indicate the likely coordination number of the metal in each of the following complexes: (a) \(\left[\mathrm{Rh}(\mathrm{bipy})_{3}\right]\left(\mathrm{NO}_{3}\right)_{3}\) (b) \(\mathrm{Na}_{3}\left[\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2} \mathrm{Cl}_{2}\right]\) (c) \(\left[\mathrm{Cr}(\mathrm{o}-\mathrm{phen})_{3}\right]\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{3}\) (d) \(\mathrm{Na}_{2}[\mathrm{Co}(\mathrm{EDTA}) \mathrm{Br}]\)

Short answer

In summary, the likely coordination numbers for the given complexes are: (a) 6, (b) 6, (c) 6, and (d) 7.

Step-by-step solution

(a) Determine Ligands and Metal

In the complex \([\mathrm{Rh}(\mathrm{bipy})_{3}]\left(\mathrm{NO}_{3}\right)_{3}\), the metal is Rh, and the ligands are bipy (2,2'-bipyridine).

(a) Coordination Number

Since each bipy ligand acts as a bidentate ligand, meaning it attaches to the metal at two points, the coordination number is 2 * 3 = 6.

(b) Determine Ligands and Metal

In the complex \(\mathrm{Na}_{3}\left[\mathrm{Co}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2} \mathrm{Cl}_{2}\right]\), the metal is Co, with two types of ligands: \(C_2O_4\) (oxalate) and Cl.

(b) Coordination Number

Each oxalate ligand acts as a bidentate ligand, while each Cl ligand occupies one coordination site. Thus, the coordination number is 2 * 2 + 2 = 6.

(c) Determine Ligands and Metal

In the complex \([\mathrm{Cr}(\mathrm{o}-\mathrm{phen})_{3}]\left(\mathrm{CH}_{3}\mathrm{COO}\right)_{3}\), the metal is Cr, and the ligands are o-phen (ortho-phenanthroline).

(c) Coordination Number

Each o-phen ligand acts as a bidentate ligand. Therefore, the coordination number is 2 * 3 = 6.

(d) Determine Ligands and Metal

In the complex \(\mathrm{Na}_{2}[\mathrm{Co}(\mathrm{EDTA}) \mathrm{Br}]\), the metal is Co, and the ligands are EDTA and Br.

(d) Coordination Number

The EDTA ligand acts as a hexadentate ligand, meaning it attaches to the metal at six points, while each Br ligand occupies one coordination site. Thus, the coordination number is 6 + 1 = 7. So, the likely coordination numbers for each complex are: (a) 6 (b) 6 (c) 6 (d) 7

Key concepts

Ligands in Coordination Compounds

In chemistry, coordination compounds consist of a central metal atom or ion bonded to one or more molecules or ions called ligands. These ligands can donate electron pairs to the metal to form coordinate covalent bonds, stabilizing the overall structure of the complex.

Ligands come in various forms - they can be neutral molecules, anions, or cations, and they vary in the number of bonds they can form with the metal center. This characteristic is crucial because it determines the coordination number of the metal, which is the total number of attachment points the metal has with the ligands.

For instance, in the complex \(\left[\mathrm{Rh}(\mathrm{bipy})_{3}\right]\left(\mathrm{NO}_{3}\right)_{3}\), the ligand bipy is a neutral molecule acting as a bidentate ligand, which means it forms two bonds with the Rhodium (Rh) metal. Understanding how these ligands interact with the metal is fundamental for predicting the structure and reactivity of coordination compounds.

Bidentate Ligands

Bidentate ligands are a fascinating class of ligands because they have the ability to form two coordinate covalent bonds with a single metal center. This 'two-toothed' attachment is made possible by the ligand having two donor atoms, which are typically nitrogen, oxygen, or sulfur atoms, capable of sharing a pair of electrons with the metal.

The way bidentate ligands clamp onto the metal atom increases the stability of the complex ion. Examples of bidentate ligands include ethylenediamine, oxalate, and 2,2'-bipyridine. In our exercise examples, both bipy and oxalate ligands display bidentate behavior. With two points of attachment for each ligand, their presence significantly influences the geometry and coordination number of the metal center.

Complex Ion Structure

Complex ions are species where a central metal atom or ion is surrounded by a number of ligands. The three-dimensional arrangement of these ligands around the metal center is known as the complex ion structure, which can significantly vary leading to fascinating geometries such as octahedral, tetrahedral, square planar, among others.

The coordination number largely dictates the geometry of the complex ion. For most transition metals with a coordination number of six, the resulting geometry is often octahedral, as seen in the solutions for our complexes (a), (b), and (c). However, for complex ion (d) with a coordination number of 7, predicting the exact geometry becomes more complex and is beyond the scope of simple rules, requiring advanced bonding theories to explain.

The structure of complex ions is not just academically interesting; it plays a crucial role in the physical and chemical properties of coordination compounds. These properties are essential in many fields, including bioinorganic chemistry, materials science, and medicinal chemistry.