Coordination numbers indicate the number of bonds a central metal ion has in a compound, ranging from 2 to 16. Transition metals are commonly used and ligands determine the coordination number. Compounds with coordination numbers can be referred to by the number or a geometric name.
Defined by Alfred Werner in 1893, a coordination number is a term used in chemistry to indicate the number of bonds a central metal ion has in a coordination compound. Coordination numbers range from two to 16, with four and six being the most common. While normally used for transition metal atoms in a compound, they can also refer to non-metallic substances.
Neutral atoms – which are atoms that have no charge – or cations – atoms with a positive charge – can both be the central atom in a coordination compound. These atoms are usually transition metals, which are those elements that react with halogens. Iron, copper, gold and chromium are all examples of transition metals.
Transition metals are listed in groups three to 12 on the periodic table of elements. Their reaction with other substances is usually obvious because solutions and compounds undergo a color change. For example, ferric chloride (FeCl) is usually green-black, but turns yellow when dissolved in a solution. Conversely, halogens include fluorine, chlorine, and iodine. They always have seven valence electrons, or electrons capable of sharing bonds with other atoms.
Attached to the metal ion is a ligand. An atom or molecule, a ligand can be neutral or negatively charged, an anion. The binders are usually halogens.
It is the attachment of ligands to the parent metal ion that determines the coordination number. For example, Ag(NH3)2+, or diaminosilver ion, is a simple coordination compound with coordination number two. Ag, or silver, is the central metal ion. Attached to the silver atom are two molecules of ammonia, NH3, one on each side of the atom. The plus sign in the chemical name shows that the compound is positively charged.
Coordination compounds are often used as catalysts or substances that initiate or change the rate of a chemical reaction. The actual compound can be positively charged, negatively charged, or neutral. Compounds with coordination numbers can be referred to by the actual number or by a geometric name that reflects both the coordination number and the basic form of the compound.
For example, a compound with a coordination number of two will be linear, so these compounds are called collinear. Collinear compounds are rare in metals, but relatively common for nonmetals. The coordination numbers of three are called trigonal planar. These are rare, but are found when the ligands are particularly large. Tetrahedral compounds have coordination numbers of four, while fives are trigonal-bipyramidal or square-pyramidal.
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