Question

What is a coordination number? What structures are generally observed for complexes in which the central metal ion has a coordination number of \(4 ?\) What is the most common structure observed for coordination number 6 ?

Short answer

Coordination number is the number of ligands bonded to a central metal ion. For coordination number 4, tetrahedral and square planar structures are common, while octahedral structure is most common for coordination number 6.

Step-by-step solution

Understanding Coordination Number

The coordination number of a central atom in a molecule or crystal is the number of atoms, molecules, or ions bonded to it. In coordination chemistry, it refers specifically to the number of ligands attached to a central metal ion.

Common Structures for Coordination Number 4

For a coordination number of 4, the most common geometries observed are tetrahedral and square planar. The type of geometry depends on several factors, including the electronic configuration of the metal ion and the types of ligands.

Most Common Structure for Coordination Number 6

When the coordination number is 6, the most widely observed geometry is octahedral. This arrangement is due to the spatial orientation that allows for six ligands to optimally coordinate around a central metal ion.

Key concepts

Coordination Chemistry

Coordination chemistry plays a crucial role in understanding the behavior and properties of complexes, which consist of a central metal atom or ion surrounded by molecules or ions called ligands. The ligands donate their lone pair of electrons to form coordinate covalent bonds with the metal center, establishing the coordination complex. The number of these bonds or interactions is known as the coordination number, which significantly influences the complex's geometry, reactivity, color, and magnetic properties. The ligands can vary widely from water molecules and halide ions to more complicated organic compounds, each bringing unique properties that affect the overall structure of the coordination complex.

Understanding coordination chemistry is not only fundamental to the field of inorganic chemistry but also has practical implications in various industries, including pharmaceuticals, catalysis, and materials science.

Tetrahedral Geometry

Tetrahedral geometry is a key spatial arrangement in coordination chemistry where a central metal ion is bonded to four ligands at the corners of a tetrahedron. Imagine a pyramid with a triangular base – the metal ion sits in the middle, equally spaced from each of the four corners. This arrangement minimizes repulsion between the ligands, thereby adopting a stable configuration.

Tetrahedral complexes arise particularly when the central metal has a coordination number of 4. It's common in complexes where the metal ion has a relatively small ionic charge and the ligands are not bulky, allowing them to fit comfortably around the metal ion without too much crowding.

Square Planar Geometry

Square planar geometry is another shape observed for complexes with a coordination number of 4, but it's less common than tetrahedral geometry. In this structure, the metal ion is at the center of a square, with one ligand at each corner. This geometry is typically seen in d8 metal ions, such as nickel(II), palladium(II), platinum(II), and gold(III), where specific electronic configurations and ligand types favor the square planar arrangement over the tetrahedral.

The square planar geometry is particularly important in certain reactions in organic chemistry, such as oxidative addition and reductive elimination, and is also characteristic of many metal-based drugs, including the well-known cancer treatment cisplatin.

Octahedral Geometry

Octahedral geometry is the most common geometry for complexes with a coordination number of 6. This geometry features a central metal ion surrounded by six ligands positioned at the vertices of an octahedron. An octahedron looks like two square-based pyramids stuck together at their bases, with the metal ion at the center. Each ligand is equidistant from the central ion, creating an optimal spatial arrangement that minimizes repulsion among the ligands and allows for a high degree of symmetry.

This arrangement is favored because of the spherical symmetry of d-orbitals, accommodating six pairs of electrons from the ligands more comfortably. Octahedral complexes are significant in various chemical processes such as catalysts in industrial processes, coloration in gemstones, and functioning of many metalloenzymes in biological systems.