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30th Apr 2021 @ 5 min read
As the name suggests, the coordination compounds are compounds involving coordination bonds in their formation.
So what is a coordination bond?
It is a bond formed between two chemical species where one specie donates one or more lone pair of electrons while the other specie accept it. These species are known as lone pair donor and lone pair acceptor respectively and the bond formed between the two is also called coordination covalent bond.
But we also know from acid-base theory that, in coordination covalent bond the lone pair donor is called Lewis base while the lone pair acceptor is termed Lewis acid. So we can also say that,
Coordination covalent bond or coordination bond is a chemical bond formed between Lewis acid and Lewis base and consequently the compounds containing these bonds are coordination compounds. Generally the specie accepting the lone pairs of electrons is a metal, which we call central metal atom. Hence we will use the word “Central Metal Atom (CMA)” whenever we talk about the lone pair acceptor or Lewis acid. Similarly the term “ligand” will be used to talk about lone pair donor or Lewis base
Finally we can define coordination compounds as,
The compounds containing coordination covalent bonds between central metal atom and ligand, where ligand donates one or more lone pairs of electrons to central metal atom
Central Metal Atom (CMA): The metallic specie which has the capability to accept one or more lone pairs of electrons while forming a coordination covalent bond.
Ligand: The chemical specie which has the capability to donate one or more pairs of electrons while forming a coordination covalent bond.
So, why the metal atoms in coordination compounds are so good at accepting lone pairs of electrons while generally metals have the tendency to release (lose) electrons?
The answer is, the metal atoms involved in forming coordination compounds are mostly transitional metals. These transitional metals have vacant d-orbitals in them. These d-orbitals are in urgent need of electrons to satisfy their electronic configuration and hence are responsible for acquiring of lone pairs of electrons by the metal atom
Examples of coordination compounds:
So what are the central metal atoms and ligands in the above example? Let’s take a deeper look
From above table we can conclude that in order to find CMA and ligand, we have to take a look at the entity inside the square brackets (of the formula of coordination compound). Inside that bracket, the metallic specie is the central metal atom while the other specie (other than CMA) is the ligand
Other terminologies are:
Coordination Entity: The entity inside square brackets (of the formula of coordination compound) is called coordination entity.
Hence in the above table are coordination entities of their respective compounds
Coordination Number: The number of lone pairs (of electrons) donated by ligand to central metal atom is termed coordination number.
For example in can donate a lone pair of electrons to central cobalt atom. As a total of 6 lone pairs are donated by NH3 to Co hence the coordination number of this compound is 6.
Note: The coordination number is not necessarily the number of ligands attached to the central metal atom as in the above example. Coordination number may vary depending upon the type of ligand attached.
Let’s take as an example. As there are two C_2 O_4 and two Cl the coordination number appears to be 2 + 2 = 4. But this isn’t the case as each C2O4 can donate two lone pairs (instead of one) and two C_2 O_4 will donate 4 lone pairs. Hence the coordination number will be 4 + 2 = 6 while the number of ligands attached to CMA is 4.
Coordination number also depicts the total number of sigma bonds formed in the coordination compound
On the basis of number of donating sites, the ligands can be identified as:
i. In both carbon and nitrogen are capable of donating a lone pair of electrons but only carbon will donate as it is less electronegative than nitrogen
ii. In both nitrogen and oxygen with negative charge (on the left side) are capable of donating a lone pair of electrons. But only the oxygen with negative charge will donate a lone pair as it has higher electron density than nitrogen. Note that the oxygen with two lone pairs (on right side) will not donate any lone pair because its octate is complete
The examples above are of ambidentate ligands
Coordination compounds can be classified in two ways
1) On the basis of ligands
2) On the basis of charge
Either way the coordination compounds can be classified further which is illustrated below
Let’s this classification in detail
1) Classification on the basis of ligand
On the basis of ligands, coordination compounds can be classified as:
a) Homoleptics: When all ligands attached to the CMA are of the same type. For example is a homoleptic. Because for our central metal atom Fe there is only one type of ligand i.e. CN . There can be multiple ligands of the same type, for example, 6 CN are attached to Fe in the above example.
b) Heteroleptics: When ligands attached to CMA are of different kinds. For example is a heteroleptic coordination compound. Because for our central metal atom Cu there are three different ligands, i.e. NH3, CL and Br.
2)Classification on the basis of charge
This classification is based on the nature of charge on coordination entity. As the charge can either be positive or negative or no charge at all, the compounds are classified as:
a) Positively Charged Complex:
In these compounds the coordination entity is positively charged. For example in the coordination entity is positively charged i.e.
b) Negatively Charged Complex:
Here the coordination entity is negatively charged. For example in the coordination entity is negatively charged i.e.
c) Neutral Complex:
These type of coordination compound the coordination entity is neutral, i.e. the charge on coordination entity is zero. For example in the charge on coordination entity is zero.
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