Coordination Compounds

The figure shown below gives information of the various terms used for defining the co-ordination compounds.




IUPAC Nomenclature for Co-Ordination Compounds


1. Positive part of a co-ordination compound is named first and is followed by the negative part.

2. In naming the complex ion, the name of the ligands are given in alphabetical order regardless of their charge followed by metal.

3. When there are several ligands of the same kind, we normally use the prefixes di, tri, tetra, penta and hexa to show the number of ligands of that type. An exception occurs when the name of the ligand includes a number (di, tri etc). To avoid confusion in such cases bis, tris and tetrakis are used instead of di, tri and tetra and the name of the ligand is placed in brackets.

4. i) Negative ligands end in 'O', for example:
F^- - Fluoro H^- - hydrido OH^- - hydroxo
Cl^- Chloro I^- - iodo NO₂^- - nitro.

ii) Neutral groups have no special endings, Examples include NH₃ ammine, H₂O aqua, CO carbonyl and NO nitrosyl. The ligands N₂ and O₂ are called dinitrogen and dioxygen. Organic ligands are usually given their common names for example phenyl, methyl, ethylenediamine, pyridine, triphenylphosphine.


iii) Positive ligands end in - ium e.g. NH₂-NH₃⁺ hydrazinium.
The spellings of ammine with two m's distinguised it from organic amines.


5. The oxidation state of the central metal is shown by Roman numeral in brackets immediately following its name (i.e. no space, e.g. titanium(IV) ).


6. Sometimes a ligand may be attached through different atoms. Thus M-NO₂ is called nitro and M-ONO is called nitrito. Similarly SCN group may bond M-SCN thiocyanato or M-NCS isothiocyanato. These may be named systematically thiocyanato-S or thiocyanato-N to indicate which atom is bonded to the metal. This convention may be extended to other cases where the mode of linkage is ambiguous.


7. When writing the formula of complexes, the complex ion should be enclosed by square brackets. The metal is named first, then the co-ordinated groups are listed in the order; negative ligands, neutral ligands; positive ligands (and alphabetically according to the first symbol with in each group).


8. If a complex contains two or more metal atoms, it is termed polynuclear. The bridging ligands which link the two metal atoms together are indicated by the prefix . If there are two or more bridging groups of the same kind, this is indicated by di-, tri- etc. Bridging groups are listed alphabetically with the other groups unless the symmetry of the molecule allows a simpler name. If a bridging group bridges more than two metal atoms it is shown as ₃, ₄, ₅ or ₆ to indicate how many atoms it is bonded to.

9. The complete metal name consists of the name of the metal, followed by-ate if the complex is an anion, which in turn is followed by the oxidation number of the metal, indicated by roman numerals in parenthesis. (An oxidation state of zero is indicated by 0 in parenthesis). When there is a latin name for the metal, it is used to name the anion (except for mercury). These names are given in the following table.

English name	Latin name	Anion name
Copper	Cuprum	Cuprate
Gold	Aurum	Aurate
Iron	Ferrum	Ferrate
Lead	Plumbum	Plumbate
Silver	Argentum	Argentate
Tin	Stannum	Stannate

Bonding in Co-Ordination Compounds


Pauling proposed a simple valence bond theory to explain bonding in Co-ordination Compounds According to this theory:


1. Central metals loses a requisite no. of electrons to form the ion.


2. The cation orbitals hybridize to form a new set of equivalent hybridized orbitals with definite directional properties.


3. Each ligand contains a lone - pair of electrons. A covalent bond is formed by the overlap of a vacant hybridized metal orbital and a filled orbital of the ligand.


4. If the complex have unpaired electron then it will be paramagnetic if not then it will be diamagnetic.


To understand the valence bond concept let us take some examples of co-ordination compounds.

1. [Cr(NH₃)₆]³⁺, the central metal is in +3 oxidation state


Orbitals of Cr³⁺ion                            


d²sp³ hybridizedorbitals of Cr³⁺       


d²sp³hybrid orbitals of [Cr(NH₃)₆]³⁺    



2. [Ni(CN₄)]^2-, Ni is in +2 oxidation state


atomic orbitals of Ni                      

dsp² hybridized orbitals of Ni⁺²   

hybridized orbitals of [Ni(CN)₄]^2-        


State of Hybridisation and Magnetic Behaviour of Some Co-Ordination Complex

Metal Complex	Metal ion	Config-uration of metal ion	Hybridisation of metal ion orbitals for ligand bonds	Number of unpaired electrons	Magnetic behaviour
[V(H2O)6]3+	V3+	d2	d2sp3	2	Paramagnetic
[Cr(NH3)6]3+	Cr3+	d3	d2sp3	3	Paramagnetic
[Mn F6]3-	Mn3+	d4	sp3d2	4	Paramagnetic
[Mn(CN)6]3-	Mn3+	d4	d2sp3	2	Paramagnetic
[Fe(CN)6]3-	Fe3+	d5	d2sp3	1	Paramagnetic
[FeF6]3-	Fe3+	d5	sp3d2	5	Paramagnetic
[FeCl4]2-	Fe2+	d6	sp3	4	Paramagnetic
[CoF6]3-	Co3+	d6	sp3d2	4	Paramagnetic
[Co(NH3)6]3+	Co3+	d6	d2sp3	0	Diamagnetic
[Ni(Cl)4]2-	Ni2+	d8	sp3	2	Paramagnetic
[Ni(CN)4]2-	Ni2+	d8	dsp2	0	Diamagnetic
[CuCl4]2-	Cu2+	d9	dsp2	1	Paramagnetic
[Zn(NH3)4]2+	Zn2+	d10	sp3	0	Diamagnetic