## Hw 17.37

Sunjum Singh 1I
Posts: 30
Joined: Fri Apr 06, 2018 11:05 am

### Hw 17.37

I'm totally lost on how to find the coordination numbers on these problems. Does anyone have a method or a way they can help me to solve these?
a) [NiCl4]^2-
b) [Ag(NH3)2]
c) [PtCl2(en)2]^2+

Kuldeep Gill 1H
Posts: 44
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### Re: Hw 17.37

Hi, the coordination number is just the number of ligands you have connected to the central atom. So for part A you have 4 Cl forming one bond each for the one Ni so you would have 4 bonds on Ni so the coordination number is 4. An easy way to figure this out is to draw the Lewis structure. Hope that helps!

Chem_Mod
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### Re: Hw 17.37

When determining the coordination number of a metal center you need to count the number of ligand bonds to the metal center. For part a) Cl is a monodentate ligand, meaning it will only bind once to the metal center. This means that there are four ligand bonds to the Ni center, so the coordination number is 4. For part b) NH3 is also a monodentate ligand, so it will only bind in one place to the metal. This means there are going to be only 2 ligand bonds on the Ag center, so the coordination number is going to be 2. For part c) this is where the distinction of the number of ligand bonds and not just the number of ligands is important. For parts a) and b) the answers were both equivalent to the number of ligands that are given in the formula, ie in a) there are 4 Cl ligands and the coordination number is 4, and in b) there are 2 NH3 ligands and the coordination number is 2. However, this only works for a) and b) because these ligands are monodentate. In part c) en is the abbreviation for ethylenediamine. This ligand has the formula H2N(CH2)2NH2 and will look like this:

The lone pair on each N are able to bind and donate to the metal center simultaneously. This means that this ligand is bidentate, which means the ligand binds (or bites) in two places on one molecule at the same time. Since the ligand is bidentate each ligand will contribute 2 bonds to the metal center, and since there are 2 en ligands they will contribute a total of 4 bonds to the Pt center, plus the two bonds from each of the 2 monodentate Cl ligands gives a total cooridination number of 6.
To do any of these problems you need to know if a ligand will be monodentate, bidentate, tridentate, or any other polydentate. If you are unsure you can draw out the ligand's Lewis structure, if it has more than one spot of electron binding in the molecule that are at appropriate angles to bind simultaneously then it will not be monodentate and you can't just add up the number of ligands in the complex to get the coordination number.

Kuldeep Gill 1H
Posts: 44
Joined: Fri Apr 06, 2018 11:02 am
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### Re: Hw 17.37

This makes a lot of sense but what would be considered an appropriate angle?