Introduction
Coordination chemistry is the study of a class of compounds formed by metals. For example, when an excess of aqueous potassium cyanide is added to aqueous ferrous sulphate, a yellow solution is formed, denoted by the following equation.
The product [Fe(CN)6]4- is called a metal complex ion; it can be isolated as its potassium salt K4[Fe(CN)6]. This product is called a coordination compound. The formation of a coordination compound from a metal is called complexation. The [Fe(CN)6]4- ion present in K4[Fe(CN)6] is called the complex species or the complex entity. The formula of the complex species is written within square brackets [ ].
Characteristics of Coordination Compounds
A coordination compound is a compound of a metal with a certain number of species called ligands bound to the metal. In [Fe(CN)6]4-, the six CN groups are called the ligands. The word ligand means that which binds or gets attached. In this example, the ligand is an anion. Many of the ligands are anions; for example, CN–, Cl– and SCN–. Neutral molecules such as H₂O, CO and NH3 can also behave as ligands. An organic molecule (such as ethylene diamine, H₂NCH₂CH₂NH₂, or an organic ion, such as the oxalate ion (–OOC-COO–), can also function as a ligand. Usually, a positive ion cannot function as a ligand, as the availability of an electron pair on the ligand for binding with a positive metal is the prime requirement for complex formation. A ligand is nucleophilic.
Complex species are of different types; cationic, neutral and anionic. Examples are [Pt(NH3)4]2+ cationic, [Pt(NH3)2Cl₂] neutral and [PtCl4]2- anionic.
Complexation of a metal ion usually increases the metallic species’ stability; thus, a metal ion’s complex is more stable than the metal ion itself. However, different complexes of metal have different stabilities.
Complex formation is often accompanied by striking changes in colour. For example, the complexation of the light yellow, aqueous Fe(III) with SCN produces a blood-red colour.
The molecules (or ions) of complexes have characteristic shapes such as tetrahedral, square planar or octahedral depending on the number of ligands attached to the central metal and the hybridisation on the metal.
A complex may be diamagnetic or paramagnetic depending on the type of bonding present in it.
A metal complex differs from an ordinary salt in some respects. When a simple salt such as FeSO4 is dissolved in water, it produces the simple constituent ions, Fe2+ and SO42-, but when a complex is dissolved in a solvent, it produces complex ions. For example, the complex K4[Fe(CN)6] produces [Fe(CN)6]4-, and K+ ions on dissolution in water. The ligands (CN–) attached to the central metal ion are not easily ionised from the metal ion. The species [Fe(CN)6]4-, though containing a metal, migrates to the positive electrode (anode) when its solution is electrolysed.
Some metal complexes are soluble in water, whereas others are insoluble. For example, [Ag(NH₂)2]+ is soluble, whereas the complex of Ni(II) with dimethylglyoxime is insoluble in water. Some complexes are soluble in organic solvents; for example, bis(acetylacetonate)copper(II) is insoluble in water but soluble in organic solvents.
Double Salts and Coordination Compounds
Ferric alum (NH4)₂SO4. Fe2(SO4)3. 24H₂O is called a double salt because it is an addition compound of two stable compounds (ammonium sulphate and ferric sulphate). When crystals of ferric alum are dissolved in water, the solution shows the properties of NH4+, SO42- and Fe3+ ions. The double salt, on dissolution, breaks down to its component simple ions. Such a salt exists only in a solid state.
However, potassium ferrocyanide K4[Fe(CN)6] does not break down into simple ions when dissolved in water. [Fe(CN)6]4- which is a complex ion that does not break down to Fe2+ and CN– ions in solution. Therefore, K4[Fe(CN)6] is called a coordination compound. Metal ions, in particular the transition metal ions, form many complexes. The chemistry of these metal ions in solution is the chemistry of their complexes. Even in the aqueous solution. Several ions exist as coordinated ions, with the H₂O molecules acting as ligands. For example, an aqueous solution of copper sulphate contains the complex ion [Cu(H₂O).
Coordination Number
The total number of monodentate ligands attached to the central metal in a complex is called the coordination number. For example, in the complex ion [Cu(NH3)4]2+, the coordination number is four as four monodentate ligands are attached to the central metal. The coordination number equals the number of sigma bonds between the ligands and the central atom. The coordination number of Cr in [Cr(C₂O4)3]3 is six though there are only three ligands since each oxalate ion forms two bonds with the chromium ion.
Coordination numbers from two to nine are known in complexes. Of these, 4 and 6 are the most common. The coordination number 3 is rare.
| Ion | Coordination Numbers | Examples |
| Ag+ | 2 | [Ag(NH3)2]+ |
| Au+ | 2 | [AuCl₂]- |
| Tl+ | 2 | [TIBr₂]- |
| Cu+ | 2,4 | [CuCl₂]-, [Cu(CN)4]3- |
| Hg2+ | 3 | [HgI3]- |
| Au³+ | 4 | [AuCl4]- |
| Zn²+ | 4 | [Zn(CN)4]2- |
| Pd²+ | 4 | [PdCl4]2- |
| Pt²+ | 4 | [PtCl4]2- |
| A1³+ | 4,6 | [Al(OH)4]-, [Al(H₂O)6]³+ |
| Cu²+ | 4,6 | [Cu(NH3)4]2+, [Cu(H₂O)6]²+ |
| Co²+ | 4,6 | [(CoCl4]2-, (Co(NH3)6]2+ |
| Ni²+ | 4,6 | [Ni(CN)4]2-, [Ni(H₂O)6]2+ |
| Feº | 5 | [Fe(CO)5] |
| Ca²+ | 6 | [Ca(H₂O)6]³+ |
| Fe²+ | 6 | [Fe(CN)6]4- |
| Fe³+ | 6 | [Fe(CN)6]3- |
| Cr³+ | 6 | [Cr(NH3)6]3+ |
| Co³+ | 6 | [Co(NH3)6]3+ |
| Pt4+ | 6 | [PtCl6]2- |
| Pd4+ | 6 | [PdF6]2- |
| Tl3+ | 6 | [TlCl6]3- |
| Zr4+ | 7 | [ZrF7]³- |
| Mo4+ | 8 | [Mo(CN)8]4- |
| Re7+ | 9 | [ReH9]2 |
Oxidation Number
The oxidation number denotes the charge the central metal atom would possess if all the ligands and the complex were removed and their electron pairs shared with the central atom. A Roman numeral represents it. For example, if all four Cl– ligands are removed from [NiCl4]2-, then the central atom Ni would have a charge of +2. Then, the oxidation number of this metal in this complex is written as NiIICl42-.
| Complex | [Co(NH3)6]3+ | [Fe(CO)5] | [Zn(CN)4]2- | [Ag(CN)2]- |
| Coordination number | 6 | 5 | 4 | 2 |
| Oxidation number | +III | 0 | +II | +I |
