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Coordination Compound

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Coordination compounds are a very important class of compounds. Many reactions in industry are carried out using highly selective homogeneous catalysts, which are coordination compounds. Coordination compounds include many classes of transition metal and rare earth compounds complexes, chelates, clusters, organometallic compounds including metallobiochemisl systems, crystals, alloys and solid solutions chemisorbed surface states and so on.


In coordination chemistry a structure is first described by its coordination number, which is the number of ligands attached to the metal. Coordination numbers are normally between two or nine but larger numbers of ligands are not uncommon for the lanthanides and actinides.

Coordination Compound Definition

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"A coordination compounds is formed by the combination of two or more stable chemical species which retains its identity in the solid as well as in the dissolved state. A coordination compounds contains a central metal ion or cation which is attached with a fixed number of anions or neutral molecules called ligands."

The compounds in which the central atom or group of atoms is surrounded by anions or neutral molecules called Coordination compounds. Coordination compounds are a very important class of compounds. Many reactions in industry are carried out using highly selective homogeneous catalysts, which are coordination compounds.

Naming Coordination Compounds

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Coordination compounds became too numerous for chemists to keep track of all the common names. To solve the nomenclature problem the international union of pure and applied chemistry (IUPAC) created a systematic procedure for naming coordination compounds.

The fundamental and basic rules for Coordination Compound Nomenclature are summarized below.

1. Order of naming ions

The positive ion is named first followed by the negative ion regardless whether the cation or anion is simple or complex. In case of complex cation ii should be written first followed by the name of anion. For non ionic or neutral complex the name of the complex is written in one words.

2. Naming of Coordination sphere

In naming the complex, the names of ligands are written first followed by the name of the central metal ion. The oxidation number of the central metal atom is expressed by Roman numeral in parentheses just after the name of central metal atom.

3. Naming of ligands

The central metal ion in a complex may be surrounded by positive, negative or neutral ligands.

For the ligands carrying a negative charge the name has a characteristic ending in "O". Some of them are listed below.

Ligand
Name of the ligand
F- Fluoro
Cl-
Chloro
Br-
Bromo
I-
Iodo
H-
Hydrido
OH
Hydroxo
O2-
Oxo
SCN-
Thicyano
O22-
Peroxo
HS-
Mercapto
S2-
Thio
CN-
Cyano
CO32-
Carbonato
NH2-
Amido
NH2- Imido
NCS- Isothiocyanato
NO2-
Nitro
N3- Nitrido
N3- Azido
C2O42-
Oxalato

For the ligand carrying positive charge the name of ligand has a characteristic ending in "lum". For example,

Ligand Name of the ligand
[NH2-NH3]+ Hydrazinium
NO2+ Nitronium
NO+
Nitrosonium

The neutral groups have no special endings.

Ligand
Name of the ligand
NH3 Ammine
H2O
Aquo or Aqua
NO
Nitrosyl
CO
Carbonyl

For organic ligands their names are used as such.

Ligand
Name of the ligand
C6H5 Phenyl
C5H5N
Pyridine
P(C6H5)3
Triphenylphosphine

4. Order of naming ligands

According to the old conversions the names of more than one type of ligands surrounding the central atom are written in the following order.
  • Negative ligand
  • Neutral ligand and
  • Positive ligand
For the ligands of the same kind present in the coordination sphere the names are written in order of their complexity or alphabetical order.

5. Use of numerical prefixes

The number of each kind of ligands is specified by di, tri, tetra etc, when there are several ligands of that kind.

6. Ending of the name of complex

The names of complex cations and neutral molecules have no distinguishing termination, but in case of complex anion the suffix "ate" is attached to the name of central metal ion.

7. Naming poly nuclear complexes

When a complex compound contains two or more metal atoms, it is termed as a poly nuclear complex. The bridging ligands which link two metal atoms together are indicated by the prefix $\mu$. When there are two or more than two bridging groups of the same kind this is indicated by di-$\mu$, tri-$\mu$ etc.

Coordination Compound Examples

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Some of the examples of coordination compounds are given below.

Solved Examples

Question 1: Determine the formula for the following coordination compound.
trans-tetraaminedichlorocobalt (III) chloride
Solution:
 
First determine the number of each type of ligand,
tetraammine = 4NH3
dichloro = 2Cl-
Next identify the metal and its oxidation state CO3+
Finally calculate the charge on the complex ion to determine the number of chloride counter ions present. Ammonia is neutral but each chloride ligand contributes a -1 charge to the complex. Overall then the complex ion has charge
[(1+3) + 2(-1)] = +1. Therefore one chloride anion is required to give a neutral salt.
[trans-Co(NH3)4 Cl2]Cl. This example also shows that a particular isomer is indicated by an italicized prefix.

 

Question 2: Identify the formula for the coordination compound given below.
tris(ethylenediamine)maganese(II) sulfate
Solution:
 
Tris(ethylenediamine) = 3H2NCH2CH2NH2 ligands. Conventionally the formula of the ligand is abbreviated en.
The metal is Mn2+. Because the en ligands are neutral the complex ion has a +2 charge requiring one sulfate anion for overall electrical neutrality.

 

Coordination complexes are also called coordination compounds. Coordination complexes are formed by coordinate bonds in which a pair of electrons is in some degree transferred from one interactant to the other. The number of bonds from the metal ion to the ligand is called the coordination number of the complex, and the maximum coordination number is evidently the largest possible number of such bonds. The maximum coordination number is determined by the electronic structure of the metal ion numbers of 4 and 6 are most common.
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