Question

Indicate the likely coordination number of the metal in each of the following complexes (a) \(\left[\mathrm{Rh}(\text { bipy })_{3}\right]\left(\mathrm{NO}_{3}\right)_{3}\) (b) \(\mathrm{Na}_{4}\left[\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2} \mathrm{Cl}_{2}\right]\) (c) \(\left[\mathrm{Cr}(0 \text {-phen })_{3}\right]\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{3}\) (d) \(\mathrm{Na}_{2}[\mathrm{Co}(\mathrm{EDTA}) \mathrm{Br}]\)

Short answer

The coordination numbers for the central metal atoms in the given complexes are: (a) 6 for Rh (b) 6 for Co (c) 6 for Cr (d) 7 for Co

Step-by-step solution

(a) Identify the metal and ligands

In this complex, the metal atom is Rh and the ligands are bipy, which is a bidentate ligand (two donor atoms). The coordination number will be determined by the number of donor atoms from the ligands.

(a) Determine the coordination number

Since there are three (3) bipy ligands, each with two donor atoms, the total number of donor atoms is \(2\times 3=6\). Thus, the coordination number of Rh in this complex is 6.

(b) Identify the metal and ligands

In this complex, the metal atom is Co, and the ligands are C2O4 which is a bidentate ligand (two donor atoms) and Cl (one donor atom). The coordination number will be determined by the number of donor atoms from the ligands.

(b) Determine the coordination number

Since there are two (2) C2O4 ligands, each with two donor atoms, and two (2) Cl ligands, the total number of donor atoms is \(2\times 2+2=6\). Thus, the coordination number of Co in this complex is 6.

(c) Identify the metal and ligands

In this complex, the metal atom is Cr, and the ligands are 0-phen, another bidentate ligand (two donor atoms). The coordination number will be determined by the number of donor atoms from the ligands.

(c) Determine the coordination number

Since there are three (3) 0-phen ligands, each with two donor atoms, the total number of donor atoms is \(2\times 3=6\). Thus, the coordination number of Cr in this complex is 6.

(d) Identify the metal and ligands

In this complex, the metal atom is Co, and the ligands are EDTA which is a hexadentate ligand (six donor atoms) and Br (one donor atom). The coordination number will be determined by the number of donor atoms from the ligands.

(d) Determine the coordination number

The complex has one (1) EDTA ligand with six donor atoms, and one (1) Br ligand with a single donor atom, making the total number of donor atoms \(6+1=7\). Thus, the coordination number of Co in this complex is 7. In conclusion, the coordination numbers for the central metal atoms in the given complexes are: (a) 6 for Rh (b) 6 for Co (c) 6 for Cr (d) 7 for Co

Key concepts

Metal Complexes

Metal complexes are molecules that consist of a central metal atom or ion surrounded by molecules or anions known as ligands. These ligands have at least one donor atom with an electron pair used to form a coordinate covalent bond with the metal. The number of these bonds, or coordinative interactions, determines the coordination number of the central metal.

Understanding metal complexes is crucial in the field of coordination chemistry. These compounds have unique structures, properties, and are widely used in various applications, including catalysis, material science, and medicine. A notable feature of these complexes is their geometry, which can be octahedral, tetrahedral, square planar, and more, often influenced by the coordination number and the spatial demands of the ligands.

Ligands

Ligands are ions or neutral molecules that bind to a central metal atom to form a coordination compound. Their role is pivotal in determining the properties of the metal complex, such as its color, magnetic behavior, and reactivity.

Each ligand has at least one lone pair of electrons and can be classified based on the number of these pairs it donates to the metal center. Monodentate ligands donate a single pair of electrons, whereas polydentate (including bidentate) ligands donate multiple pairs. Ligands can also be classified by their charge, with negative ligands called anionic, neutral ones neutral, and positively charged ligands, though rare, known as cationic. The strength of the ligand-metal bond varies depending on the electronic nature of the ligands, which also affects the overall stability of the complex.

Bidentate Ligand

A bidentate ligand is a type of ligand that has two atoms capable of donating electron pairs to form coordinate covalent bonds with a metal atom. This 'two-toothed' binder's ability to attach to the central atom at two points increases the stability of the metal complex through the chelate effect - a phenomenon where larger rings of atoms in the complex are generally more stable due to reduced entropy of the system.

Common examples of bidentate ligands include ethylenediamine, oxalate, and 2,2'-bipyridine (bipy) as mentioned in the exercise. When forming complexes, bidentate ligands create a more fixed and defined geometry, which can lead to notable properties in coordination compounds. This can be crucial in applications like chelation therapy in medicine and in the design of metal-organic frameworks for materials science.