Inorganic Chemistry Lesson of the Day: 6-Coordinated Complexes

The most common coordination number for inorganic coordination complexes is 6, and these complexes will most commonly adopt an octahedral geometry.  This geometry is especially common for coordination complexes with a first-row transition metal ion as the Lewis-acid centre.  It consists of 4 ligands forming a plane, and 2 ligands above and below the plane.  The “octa-” prefix in “octahedral” refers to the 8 faces that this geometry has.

Two alternative geometries of 6-coordinated complexes are the trigonal prism and hexagonal plane; these are far less common than the octahedron.

Advertisements

Inorganic Chemistry Lesson of the Day – Coordination Complexes

A coordination complex is a compound that consists of Lewis bases bonded to a Lewis acid in its centre.  The charge of the complex can be neutral, positive, or negative; if the complex has a positive or a negative charge, then it is called a complex ion.  The Lewis acid is almost always a metal atom or a metal ion.  The Lewis bases are called ligands, and they are often covalently bonded to the Lewis acid.  Common ligands include carbon monoxide, water, and ammonia; what unifies them is the existence of at least one lone pair of electrons in their outermost energy level, and this lone pair of electrons is donated to the Lewis acid.

Some key terminology:

  • The donor atom is the atom within the ligand that is attached to the Lewis acid centre.
  • The coordination number is the number of donor atoms in the coordination complex.
  • The denticity of a ligand is the number of bonds that it forms with the Lewis acid centre.
    • If a ligand forms 1 bond with the Lewis acid centre, then it is monodentate (sometimes called unidentate).
    • If a ligand forms multiple bonds with the Lewis acid centre, then the coordination complex is polydentate.  For example, a bidentate ligand forms 2 bonds with the Lewis acid centre.

In later Inorganic Chemistry Lessons of the Day, I will only refer to coordination complexes with metal atoms or metal ions as the Lewis acid centres.