Polydentate structures

[Tracy Tran 3L]

Why do some lone pairs form bonds whereas other lone pairs on that same atom is unable to?

[105605391]

This could be due to the location of the lone pairs making them unable to bond. If the lone pairs are too far apart this can not occur. As well if the two lone pairs are on opposite sides and unable to rotate due to a pi bond this bonding is unable to occur.

[Anthony Tam]

I'm pretty sure on one atom that has multiple lone pairs, only one lone pair can form a coordinate covalent bond because two bonds would make the angle too acute and unstable. In order for one atom to form multiple coordinate covalent bonds, I think the cation has to be very small. This is why you need spacer atoms for polydentate ligands. For example, in ethylenediamine, the two ethylene serve as spacer atoms for the two ammines to coordinate with the transition metal (the spacer atoms make the bond angles much less acute and more stable) so that it can form two coordinate covalent bonds (acting as a bidentate).