Question

Why H2O acts as unidentate molecule even though oxygen atom has 2 lone pair of electrons ?

Answer 1

The explanation is as follows.

In H2O, both lone pairs are on the oxygen. There would be only one binding site between H2O and the central metal. So it is a monodentate.
Also, water is too small to form two bonds to a TM center, especially since both lone pairs are on the same O atom. Polydentate ligans form their multiple coordination bods through lone pairs on different atoms in their structure.

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