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In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central ion/molecule/atom is called a ligand .
The coordination geometry depends on the number, not the type, of ligands bonded to the metal centre as well as their locations. The number of atoms bonded is the coordination number. The geometrical pattern can be described as a polyhedron where the vertices of the polyhedron are the centres of the coordinating atoms in the ligands. [1]
As one example, Pauling considered the three mineral forms of titanium dioxide, each with a coordination number of 6 for the Ti 4+ cations. The most stable (and most abundant) form is rutile , in which the coordination octahedra are arranged so that each one shares only two edges (and no faces) with adjoining octahedra.
In cases where a metal-ligand bond in a coordination complex is involved, these reactions are typically organometallic in nature and involve a bond between a transition metal and a carbon or hydrogen. [1] It is usually reserved for the case where the coordination number and oxidation state of the metal remain unchanged. [2]
The steric number of a central atom in a molecule is the number of atoms bonded to that central atom, called its coordination number, plus the number of lone pairs of valence electrons on the central atom. [11] In the molecule SF 4, for example, the central sulfur atom has four ligands; the coordination number of sulfur is four. In addition to ...
The preferred ligand takes the lowest priority number. For example, of the ligands acetonitrile, chloride ion and pyridine thepriority number assigned are chloride, 1; acetonitrile,2; pyridene,3. Each coordination type has a different procedure for specifying the configuration index and these are outlined in below.
For example, NO 2 − is a strong-field ligand and produces a large Δ. The octahedral ion [Fe(NO 2 ) 6 ] 3− , which has 5 d -electrons, would have the octahedral splitting diagram shown at right with all five electrons in the t 2 g level.
This diagram is for octahedral interstices (coordination number six): 4 anions in the plane shown, 1 above the plane and 1 below. The stability limit is at r C /r A = 0.414. The radius ratio rule defines a critical radius ratio for different crystal structures, based on their coordination geometry. [1]