Question

The closed ring compounds formed by bidentate ligands on binding to metal or metal ions are called:
A. Monodentate
B. Chelates
C. Ambidentate
D. None of these

Answer 1

Hint: Recollect coordination chemistry concepts. Recollect the types of ligands and what bidentate ligand is and how can it form closed ring compounds. Take an example and then draw the structure of that complex and then determine what the complex will be known as. Take a look at all the options in the question and then decide the correct answer.

Complete step by step answer:
In coordination compounds, a central metal is bound to ligands in its coordination sphere to form a complex. Ligands can be atoms, ions, or molecules that donate electrons to the metal. And also these complexes can be neutral or charged. Now, we will understand what will happen if the complex is charged or neutral. When a complex is charged it is stabilized by neighbouring counter-ions. We should understand that, in a complex ion there is a metal ion at its centre with a number of other molecules or ions surrounding it. These ligands are attached to central metal by coordinate bonds. Some examples of coordination compounds are $Ni{{(CN)}_{4}}$, ${{K}_{4}}[Fe{{(CN)}_{6}}]$, etc.

Let’s now take a look at the question. A bidentate ligand is a ligand that has two teeth or we can say that, there are two atoms of one ligand that coordinate directly to the central atom in a complex. An example of a bidentate ligand is ethylenediamine. A single molecule of ethylenediamine can form two coordinate bonds with a metal ion. For example, ${{[Ni{{(en)}_{2}}]}^{2+}}$
The structure of ethylenediamine and its bonding is shown below:

Just look at the structure formed by this ligand. If you look closely, you will find that it seems crab’s claws are folded and holding the metal and the ligand is the crab. Basically the complex has crab-like structure so thus, it is known as chelate. Chelate means Crab-like. Chelates are complexes that are formed when polydentate ligands form complexes with central metal atoms. Some such ligands forming chelates are DMG (dimethylglyoxime) which is bidentate, dien (diethylenetriamine) which is tridentate, EDTA (ethylenediaminetetraacetic acid) which is hexadentate, etc.
So, the correct answer is “Option B”.

Note: Chelates play important roles in oxygen transport and in photosynthesis like in haemoglobin and chlorophyll. And we should know that many biological catalysts (enzymes) are chelates. And also we can say that chelates are also economically important, both as products in themselves and as agents in the production of other chemicals. There is chelation therapy to remove toxic metals from the body.