Coordination Chemistry
Coordination compounds are formed by the union of a metal ion (usually a transition metal) with a nonmetallic ion or molecule. For example, when white, anhydrous copper sulfate, CuSO4, is exposed to ammonia gas, a deep blue crystalline product is formed. This product consists of four moles of ammonia per mole of copper sulfate; the positive copper ion Cu2+ bonds to four ammonia molecules. The nitrogen atom of each NH3 molecule contributes a pair of shared electrons to form a covalent bond with the Cu2+ ion. This type of bond, where both electrons are contributed by the same atom, is referred to as a coordinate covalent bond.
The reaction of the Cu2+ ion with four ammonia molecules r esembles the bond between a proton and an ammonia molecule to form the ammonium ion. In both cases, the ammonia molecule is acting as a Lewis base, donating the pair of electrons required to form the coordinate covalent bond. The Cu2+ ion acts as a Lewis acid in accepting a pair of electrons.
Complex ions are ions that have a molecular structure consisting of a central atom bonded to other atoms by coordinate covalent bonds. Compounds containing a complex ion are known as coordination compounds, or complex compounds. In the broadest sense, complex ions are charged species consisting of more than one atom. However, in a more restricted sense, complex ions are charged species in which a metal atom is joined by coordinate covalent bonds to neutral molecules and/or negative ions.
The metal atom in the complex ion is referred to as the central atom. The molecules or anions attached to the central atom are called coordinating groups or ligands. The ligand may be either positively or negatively charged, or may be a molecule of water or ammonia. (When ammonia is the ligand, the compounds are called ammines.) L igands have electron pairs on the coordination atom that can be either donated or shared with the metal ions. The metals that most commonly form stable coordination compounds are cobalt, platinum, iron, copper, and nickel. The number of bonds formed by the central atom is referred t o as its coordination number. The coordination number is usually 2, 4, or 6, often depending on the type of ligand involved. The bonding between the ligand and the metal ion is intermediate between covalent and electrostatic. The charge on the complex ion is the sum of the charges on the metal ion and the ligands.
Besides being important in the chemical industry, coordination compounds play a primary role in sustaining life on Earth. Two well known coordination compounds in biological systems are chlorophyll and hemoglobin. Chlorophyll, the chief molecule in plant photosynthesis, is a magnesium complex; hemoglobin, a major component of animal blood that carries oxygen to cells in the body, is an iron complex.
In the chemical industry, coordination compounds are used in qualitative analysis to separate metal ions, and to identify unknown ions in solution. One test for the presence of silver ions in solution is to add chloride ions to the solution. Any silver ions present form a white precipitate (silver chloride). Addition of excess ammonia dissolves the silver chloride, causing it to forms the stable metal complex, [Ag(NH3)2]+.
The formation of a coordination compound is often accompanied by a color change. Invisible ink, for example, is an aqueous solution of CoCl2 having a very pale pink color, or being essentially colorless. If one writes with this ink on a sheet of paper, and then heats the sheet, the ink turns blue. When the paper cools, the ink once again becomes colorless (invisible). The reason for this is changes in the metal complex. In dilute solution, the cobalt forms the complex [Co(H2O)6]Cl2. With the application of heat, some of the water molecules evaporate, and a new complex, [CoCl2(H2O)2] is formed. The new complex is blue color instead of pale pink like the first. Upon cooling, water is again taken up by the complex and the original pink complex reforms.
Coordination compounds has been of interest to scientists for over 200 years. The first metal complex, discovered in 1798 by Tassaert, was hexaamminecobalt(III) chloride (CoCl36NH3. Eighteenth century chemists were unable to explain how two stable compounds, CoCl3 and NH3, could combine to form another stable complex.
Metal complexes are generally prepared by reacting a salt with another molecule or ion. The early coordination compounds prepared using ammonia were metal amines. Other early coordination compounds used the anions CN-, NO2-, and Cl-, an example being Reinecke's salt (NH4 [Cr(NSC)4(NH3)2].
As coordination chemistry evolved, other important discoveries followed. By comparing the electrical conductivitie s of solutions having the same concentration s, it was shown that the number of ions in solution could be estimated. Also, the existence of isomers of coordination compounds was verified.
Nearly 100 years after the discovery of the first coordination compound, Alfred Werner established the foundations for the modern science of coordination chemistry, with the elucidation of such concepts as primary valence and coordination number.
Although Werner's theories explained the existence and structure of coordination compounds, they did not describe the coordinate bond, or secondary valence. At least three theories have been proposed to describe the coordinate bond: valence bond theory, crystal field theory, and molecular orbital theory.
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