Why chelation increases entropy
The chelate effect is very simple. Consider the addition of $\ce{Na2H2edta}$ to a random transition metal (e.g. manganese(II)). Your starting complex is a hydrate complex; we shall assume it is $\ce{[Mn(H2O)6]^2+}$. The reaction equation is given below:

$$\ce{[Mn(H2O)6]^2+ + edta^4- &lt;=&gt;&gt; [Mn(edta)]^2- + 6 H2O}\tag{1}$$

You started with two molecular entities, you end up with seven. Since $2&lt;7$, the product side has a higher entropy (more independent particles). The same is true for any chelating ligand that displaces more than one monodentate ligand.



Yes, upon chelation the chelating ligand does lose degrees of freedom concerning bond rotation. However, the number of particles greatly outweighs that.