EDTA 6 Bonding Sites

[Tiffany Chao 2H]

I don't completely understand why EDTA 4- has 6 bonding sites. I understand that there is 2 from the 2 nitrogen atoms which each has a lone pair. However, there are 4 double bonded oxygen atoms with 2 lone pairs each and 2 single bonded oxygens with 3 lone pairs each. Why is it that the single bonded 3 lone pair oxygens count as a bonding site but not the double bonded ones? I've attached the image that I'm referring to.

Thanks!

[Angus Wu_4G]

The double-bonded oxygens do not count as ligand bonding sites because those oxygens can't serve as lewis bases. Lewis bases are electron donors and will donate a pair of electrons to form a coordinate covalent bond. Since the double-bonded oxygens already have 2 bonds and since oxygen does not form more than 2 covalent bonds, those double-bonded oxygens are not ligand bonding sites, whereas the single bonded oxygens only have one bond and are very eager to donate a pair of electrons to form that second bond and lose their negative formal charge.