Co-ordination compounds are special class of compounds in which the central metal atom is surrounded by ions or molecules beyond their normal valency. These are also referred to as coordination complexs or simply complexs. These compounds are widely present in the minerals, plants and animals and play many important functions. Many biologically important compounds are coordination compounds in which complicated organic species are bound to metal ions. The common examples are haemoglobin which is a co-ordination compound of iron, chlorophyll which is a co-ordination compound of magnesium, etc.
Co-ordination compound or complex ion: A co-ordination compound contains a central metal atom or ion surrounded by a number of oppositely charged ions or neutral molecules. For example, [Co (NH3)3Cl3] is a coordination compound, in which the cobalt (+3) ions is surrounded by three ammonia molecules and three negatively charged chloride ions. Other examples are [Ni(NH3)6]Cl2, K[Ag(CN)2], [Cu(NH3)4]SO4 etc.
From the above examples, it is clear that a coordination compound contains a complex ion. A complex ion may be defined as an electrically charged species which consists of a central metal atom or ion surrounded by a group of ions or molecules. For examples, [Ni(NH3)6]2+ is a complex ion in which the central nickel ion, Ni2+, is surrounded by six ammonia molecules. Similarly, the complex ion [Ag(NH3)2]+, consists of central silver ion, Ag+, surrounded by two ammonia molecules. Therefore, complex ion may be positively charged or negatively charged or neutral species.
Cationic complex: A complex ion which has a net positive charge is called cationic complex. For examples, [Co (NH3)6]3+ , [Ni (NH3)6]2+ , etc.
Anionic Complex: A complex ion which has a net negative charge is called anionic complex. For example, [Ag (CN)2]–, [Fe(C2O4)3]3-, [Fe(CN)6]4-, etc.
Neutral complex: A complex which has no net charged is called a neutral complex or simply a complex. For examples, [Co(NH3)3Cl3], [Ni(CO)4], [Pt(NH3)2Cl2], etc.
Central Ion and Ligands: The cation to which one or more neutral molecules or anions are attached is called the central ion while the neutral molecules or ions are called ligands. Thus, in the complex ion [Ni(NH3)6]2+, the Ni2+ ion is the central ion and the molecules of ammonia are the ligands. Similarly, in the complex ion [Co(NH3)5Cl]2+,the Co3+ ion is the central ion while the ammonia molecules and chloride ion are the ligands.
[Ni(NH3)6]2+ : Central ion = Ni+
Ligands = NH3 molecules
[Co(NH3)5Cl]2+ : Central ion = Co3+
Ligands = NH3 molecules and Cl– ion
The ligands are attached to the central metal atom or ion through coordinate bonds. It is essential; therefore, that while the central ion should have vacant orbital, the ligands should have vacant orbitals, the ligands should have lone pairs of electrons in their outermost orbitals which can be donated to the central ion. The atom in the ligands which can donate the electron pair is called donor atom or coordinating atom. For example, in ammonia, (¬: NH3 ), nitrogen is the donor atom and in water,( ¬:OH2) oxygen is the donor atom.
Types of ligands: The ligands may contain one or more than one donor atoms for coordination with the central atom. Accordingly, the ligands are classified as follows:
i) Unidentate or monodentate ligands: Ligands which can coordinate to the central ion through only one atom are as unidate or monodentate ligands. The example of unidate ligands are:
NH3, H2O, Cl–, CN, OH, etc.
ii) Bidentate ligands: Ligands which have two donor atoms and therefore, can coordinate to the central ion at two positions are called bidentate ligands. The examples of bidentate ligands are: 
NB: Oxalate ion is abbreviated as ox and ethylenediamine as en.
iii) Polydentate ligands: Ligands having more than two donor atoms present in the molecule are called polydentate ligands. These are called as tridentate (three), tetradentate (four), pentadentate (five) and hexadentate (six) ligands depending upon the number of donor atoms present in the molecules.
Chelating ligands and chelate: When a bidentate or a polydentate ligand is attached by two or more donor atoms to the same central metal ion forming a ring structure, the ligand is called chelating ligand. The complex is called chelate. For example, when a bidentate ligand such as ethylenediamine attached to Cu+2 ion through two amino groups and forms a ring structure, it is called chelating ligand. The resulting complex ion [Cu(NH2CH2CH2NH2)2]2+ or [Cu(en)2]2+ is called chelate.
Coordinatin Sphere: Central metal atom and the ligand directly bonded to it is collectively known as the coordination sphere. This part of the complex behaves as one unit and is non-ionisable. It is generally written in a square bracket [].For example, [Ag (CN)2]– represents as coordination sphere in the compound [Ag (CN)2] Cl.
Coordination number: In coordination compounds ligands are attached to the central metal atom through coordinate bonds. The total number of ligands attached to the central metal ion is called the coordination number of that ion. In other wards, coordination number is the number of ligands in the coordination sphere of the complex compound. For examples, the coordination numbers of central metal ion in different complex are:
[Ag (CN)2]– Coordination number of Ag+ = 2
[Cu(NH3)4] Coordination No. of cu+2 = 4
[Co (NH3)3Cl3] C. No. of Co3+ = 6
[Fe(C2O4)3]3- C. No. of Fe3+ = 6
Nomenclature of coordination compounds: The nomenclature system given by IUPAC has been given below:
1) Order of naming ions: In ionic complexes, the cation is named first and then the anion (as in NaCl: sodium chloride). Non ionic complexes are given a one word name.
2) Naming of coordination sphere: In naming of coordination sphere, the ligands are named first and then the central metal ion.
3) Naming of ligands: The names of negative ligands end in –o and names of positive ligands end in –ium. The neutral ligands are named as such. For example,
i) Negative ligands end in –o:
F- Fluoro CN- cyano
Cl- chloro CH3COO– acetato
Br- bromo NO2– nitro
OH- hydroxo SO42- sulphato
SCN– thiocyanato N3– azido
C2O42- oxalate NH2– amido
ClO3– chlorato NH2- imido
ClO3– chlorato CO32- carbonato
H– hydrido NO3– nitrato
ii) Positive ligands end in –ium:
NO+ nitrosonium NO2+ nitronium
NH2NH3+ hydrazinium
iii) Neutral ligands are named as such:
NH2CH2CH2NH2 ethylenediamine
C6H5N pyridine
(C6H5)3P triphenylphosphine
However, there are few exceptions in naming neutral ligands. For example,
H2O aquo NH3 ammine
NO nitrosyl CO carbonyl
4) Order of naming ligands: When more than one type of ligands are present, they are named in alphabetical order of preference without separation by hyphen. For example, in the complex [Co(NH3)4 Cl(NO2)]+, the ligands are named in the order: ammine, chloro and nitro. Similarly, in the complex K3[Fe(CN)5NO], the ligands are named as cyano and nitrosyl.
5) Numerical prefixes to indicate number of ligands: When more than one ligands of a particular kind are present in the complex, the prefixes di-, tri-, tetra-, penta-, hexa-, etc. are used to indicate their number: two, three, four, five and six respectively.
When the name of the ligand, includes the numerical prefix (di, tri, tetra), then the prefixes bis, tris, tetrakis are used for two, three, four ligands, respectively. Such ligands are called complex ligands.
For e.g. to indicate two simple ligands such as chloro, bromo, ammine, oxalate, etc. We use the prefix di but to indicate two complex ligands such as ethylnediamine we use the prefix bis (ethylenediamine). The name of the complex ligands is given in brackets.
6) Ending of names. When the complex is anionic, the name of the central metal atom ends in –ate. For cationic and neutral complexs, the name of the metal is written without any characteristic ending. For example, the cationic complex [Co(NH3)6]Cl3 is named without characteristic ending of the name of the metal as:
[Co(NH3)6]Cl3 Hexaaminecobalt (III) chloride
The coordination compound, K[Pt(NH3)Cl5] which contain the anionic complex [Pt(NH3)Cl5]– is named with ending of the name of the metal as –ate.
K[Pt(NH3)Cl5] Potassium amminepentachloroplatinate(IV)
Similarly, the anionic complex Ca2[Fe(CN)6] is named as calcium hexacyanoferrate(II).It may be noted that for anionic complexes the Latin names of certain metals are commonly used. For example,
Ferrate for Fe, cooperate for Cu, argentite for Ag, aurate for Au, stannate for Sn etc. However, if the complex is cationic the name of the metal is given as such, e.g. iron for Fe, silver for Ag, gold for Au, copper for Cu, etc.
K3[Fe(CN)6] : Potassium hexacyanoferrate (III).
[Fe(CO)5] : Pentacarbonyl iron(0).
7) Oxidation number of central metal ion: The oxidation of the central metal ion is designated by a Roman numeral (such as II, III, IV) in the brackets at the end of the name of the complex without a gap between the two. For example,
(i) [Co(NH3)5Cl]Cl2
In this case the ligands are chloro and ammine. The complex is cation and chloride is anion. The oxidation state of cobalt is III as
X + 5 (0) – 1- 2 = 0 or, x = + 3.
Its name is pentaamminechlorocobalt (III) chloride.
(ii) K3[Fe(CN)6]
In this case, the ligands are cyano. The complex is anionic. The oxidation of iron is + 3 as,
3 (+1) + x + 6(-1) = 0 or, x = +3
The name of the complex is potassium hexacyanoferrate (III). It may be noted that if the complex containing the central ion, Fe+ is anionic, the Latin name of the metal is used i.e., ferrate.
8) Bridging groups in bidentate ligands: For ligands which act as bride between two metal atoms, the Greek letter mu (μ) is written before their names. For example,
9) Point of attachment: When a ligand can coordinate through more than one atom, then the point of attachment of the ligand is indicated by putting the symbol of the atom through which coordination occurs after the name of the ligand. Sometimes, different names are used for alternative modes of attachment. For example, ─NO2– (through N): nitro, ─ONO– (through O): nitrite, ─SCN– (through S): thiocyanato, ─NCS– (through N): Isothiocyanoto.
[Co (NH3)3(NO2)3] Triamminetrinitrocobalt (III).
[Co (NH3)5(ONO)] SO4: Pentaamminenitritocobalt (III) sulphate.
K [Ag (CN)2]: Potassium dicyanoargentate (I).
K3[Cr (C2O4)3]: Potassium trioxalatochromate (III), etc.