Oxidation Numbers

joanneyseung22
Posts: 31
Joined: Fri Sep 28, 2018 12:15 am

Oxidation Numbers

How do you figure out oxidation numbers of the ligands inside of a coordination compound?
For example, in hexacyanoferrate(II) ion, or [Fe(CN)6]^4-, the CN's charge is -1 overall. However, in hexaamminecobalt(III) ion, or [Co(NH3)6]^3+, NH3's charge is 0. How can one figure all this information out?

armintaheri
Posts: 68
Joined: Fri Sep 28, 2018 12:26 am

Re: Oxidation Numbers

You're generally supposed to know the charges on common polyatomic ions. So you should know that CN has a -1 charge. If you don't know the charges beforehand, the name of the compound may also help you figure it out. The (II) in hexacyanoferrate(II) tells you that the iron has a +2 charge. Since the molecule has an overall charge of -4, and the iron is +2, then the 6 cyanides must have a combined charge of -4 -2 = -6. So each cyanide has a charge of -1. But if you're just given the formula and not the name you really have no way of figuring it out. Which is why you should make sure you know your polyatomic ions.