Question

What is the coordination number of \[Cr\] in \[{K_3}[Cr{\left( {OX} \right)_3}]\]? \[\left\{ {OX = {C_2}{O_4}^{ - 2}} \right\}\]
\[
  A.\;\;\;\;\;6 \\
  B.\;\;\;\;\;5 \\
  C.\;\;\;\;\;4 \\
  D.\;\;\;\;\;3 \\
 \]

Answer 1

Hint: Coordination number is the number of ligands present in a coordination complex attached to the central metal atom. Ligand surrounds the central metal atom in a coordination complex. Every ligand has different atoms donating their lone pairs to form bonds with the central metal atom. In the above complex ox is a bidentate ligand.

Complete answer: The coordination complex given above is \[{K_3}[Cr{\left( {OX} \right)_3}]\]
In this particular complex \[Cr\] is the central metal atom, \[K\] is the metal ion complex also called the counter ion, and \[OX\] is the ligand that surrounds the central atom \[Cr\]. The counterions are the free ions always written outside the coordination complex.
Ligands are bonded to the central metal atom with lone pairs donated by donor atoms. We call this a ligand coordinating with the central atom. The ligand can coordinate with the central metal atom with different donor atoms. There are many different types of ligands like monodentate, bidentate as well as polydentate ligand.
The compound given to us is \[{K_3}[Cr{\left( {OX} \right)_3}]\] here \[OX\] means oxalate ligand and we know that oxalate is a bidentate ligand that means it will coordinate with two donor atoms. And this ligand occupies two positions around the central metal atom. It can be seen above that there are three oxalate ligands so it will occupy \[2{\text{ }} \times {\text{ }}3{\text{ }} = {\text{ }}6\] positions around the central metal atom.
Therefore the coordination number of \[Cr\] is \[6.\]

Note:
Monodentate ligands coordinate donate one lone pair to form bond with the central metal atom e.g. \[{H_2}O,C{l^ - },{\text{ }}{F^ - }\] etc , bidentate donate two lone pairs for coordinating with central atom e.g. en(ethylenediamine), oxalate ion(ox)- \[{C_2}{O_4}^{ - 2}\] and polydentate ligands donate more than two lone pairs to coordinate with central atom e.g. EDTA(ethylenediaminetetraacetate ion)