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17.33 (whole)

Posted: Wed Nov 22, 2017 11:07 am
by ClaireHW
Hi, can someone explain how to do this problem?

Which of the following ligands can be polydentate? If the ligand can be polydentate, give the maximum number of places on the ligand that can bind simultaneously to a single metal center.
a) HN(CH2CH2NH2)2
b) Co3 2-
c) H2O
d) oxalate


(Claire Woolson Dis 3J)

Re: 17.33 (whole)

Posted: Wed Nov 22, 2017 1:51 pm
by Minie 1G
Polydentate ligands is basically when you bind a bunch of these given molecules to a central metal atom or ion. So how many bonds does each molecule make with the metal?

I'll give the easiest example, part c, which is water. This is monodentate because only the oxygen atom is a donor atom that has electron pairs able to be used for bonding to the central metal atom. An example of bidentate, oxalate, has TWO donor atoms that can bond to the central metal atom. (see on the pic, there are two potential bonds drawn to the metal, M)


But it's not just all atoms in a molecule that have lone pairs can form bonds with the same metal at the same time. Polydentate ligands can usually be recognized as having more than 2 lone pair donating sites.

Here's an example... in the following picture, M is the metal. Monodentate means one bond is formed between the molecule and metal, bi means two, and tri means 3. The top row is PF6 and on the bottom is AlCl4. These examples can be mono, bi, or tridentate! Basically, see how many donor atoms each molecule has.

Re: 17.33 (whole)

Posted: Thu Nov 23, 2017 6:02 pm
by AtreyiMitra2L
Hi! To determine if a molecule is polydentate, you have to understand the geometry and the existence of lone pairs on atoms. For a, there are three atoms that have lone pairs (3 nitrogens). Therefore, you know that the maximum number of places the ligand can bind to the metal ion is 6. B is slightly trickier. While there are 3 atoms with lone pairs, it is impossible to bind with all 3 of them at one. For them to bind with a metal ion, they have to be on the same side. But this ion is in this weird position bc of the 120 degree angle that it can be mono or bidentate. Because the angle is so large, it depends on the metal ion to know which one it is. Therefore, it can be either. In c, water is simply monodentate. The existence of lone pairs must be on separate atoms. Here, there is only one atom (O) even though there are two lone pairs. D is bidentate as there are 4 atoms with lone pairs. They can only bind to the metal ion if they are on the same side. Therefore, it is 2.

Re: 17.33 (whole)

Posted: Tue Nov 28, 2017 9:17 pm
by ClaireHW
Thanks guys! Impressive and super helpful responses.

(Claire Woolson Dis 3J)