Chemistry: The Central Science. Chapter 24: Chemistry of Coordination Compounds

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1 Chemistry: The Central Science Chapter 24: Chemistry of Coordination Compounds Metal compounds with complex assemblies of metals surrounded by molecules and ions are called coordination compounds 24.3: Nomenclature of Coordination Chemistry The rules that govern naming of complexes of substances are as follows: o In naming salts, the name of the cation is given before the name of the anion o Within a complex ion or molecule, the ligands are named before the metal Ligands are listed in alphabetical order, regardless of charge on the liquid Prefixes that give number of ligands are not considered part of the ligand name in determining the order o The names of the anionic ligands end in the letter o, whereas neutral ones ordinarily bear the name of the molecules o Greek prefixes (di-, tri-, tetra-, penta-, and hexa-) are used to indicate the number of each kind of ligand when more than one is present If the ligand itself contains a prefix of this kind (for example. ethylenediamine), or is polydentate, alternate prefixes are used (bis-, tris, tetrakis-, pentakis-, hexakis-) and the ligand name is placed in parentheses o If the complex is an anion, its name ends in ate o The oxidation number of metal is given in parentheses in Roman numerals following the name of the metal 24.4: Isomerism When two or more compounds have the same composition but a different arrangement of atoms, we call them isomers o Isomerism the existence of isomers is a characteristic feature of both organic and inorganic compounds Two kinds of isomers o Structural isomers (which have different bonds) and stereoisomers (which have the same bonds but different ways in which ligands occupy the space around the metal center) Structural Isomerism

2 o Many different types of structural isomerism are known in coordination chemistry Linkage isomerism is a relatively rare but interesting type that arises when a particular ligand is capable of coordinating to a metal in two different ways E.g. nitrite ion, NO 2 -, can coordinate through either a nitrogen or an oxygen atom o When it coordinates through the nitrogen atom, the NO 2 - ligand is called nitro o When it coordinates through the oxygen atom, it is called nitrito and is generally written ONO - Coordination-sphere isomers differ in the ligands that are directly bonded to the metal, as opposed to being outside the coordination sphere in the solid lattice E.g. there are three compounds whose chemical formula is CrCl 3 (H 2 O) 6 : o [Cr(H 2 O) 6 ]Cl 3 (a violet compound) o [Cr(H 2 O) 5 Cl]Cl 2 H 2 O (a green compound) o [Cr(H 2 O) 4 Cl 2 ]Cl 2 H 2 O (a green compound)

3 o In the two green compounds the water has been displaced from the coordination sphere by chloride ions and occupies a site in the crystal lattice Stereoisomerism o Stereoisomers have the same chemical bonds but different spatial arrangements o In square planar complex [Pt(NH 3 ) 2 Cl 2 ], the chloro ligands can be either adjacent to or opposite each other This particular form of isomerism, in which the arrangement of the constituent atoms is different through the same bonds are present, is called geometric isomerism Geometric isomers generally have different properties, such as colors, solubilities, melting points, and boiling points o A second type of stereoisomerism is known as optical isomerism Optical isomers, called enantiomers, are mirror images that cannot be superimposed on each other

4 Just as there is no way that we can twist or turn our right hand to make it look identical to our left, so also there is no way to rotate on of these enantiomers to make it identical to the other Molecules or ions that are not superimposable on their mirror image are said to be chiral o Enzymes are among the most important chiral molecules and many enzymes contain complexed metal ions Most of the physical and chemical properties of optical isomers are identical The properties of two optical isomers differ only if they are in a chiral environment that is, one in which there is a sense of right- and left-handedness Optical isomers are usually distinguished from each other by their interaction with plane-polarized light If light is polarized, the light waves are oscillating in a single plane If the polarized light is passed through a solution containing one optical isomer, the plane of polarized light is rotated to the right (clockwise) or to the left (counterclockwise) o The isomer that rotates the plane of polarization to the right is dextrorotatory; it is labeled the dextro, or d, isomer o Its mirror images rotates the plane of polarization to the left; it is levorotatory and is labeled the levo, or l, isomer Because of their effect on plane-polarized light, chiral molecules are said to be optically active

5 o When a substance with optical isomers is prepared in the laboratory, the chemical environment during the synthesis is not usually chiral Equal amounts of the two isomers are obtained; the mixture is said to be racemic A racemic mixture will not rotate polarized light because the rotator effects of the two isomers cancel each other