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In example 17.1 in the textbook, the problem asks you to name the coordination compound [Co(NH3)3(OH2)3]2 (SO4)3. It shows that the cation complex, [Co(NH3)3(OH2)3]2, has a charge of +3, but I don't understand how they got that.
I checked 17.1 in the book and it is not this problem so I don't really know where the problem is that you are talking about, but I'll try my best to help. In [Co(NH3)3(OH2)3]2 (SO4)3, (SO4)3 would have a total charge of -6. This means in order for that to balance with [Co(NH3)3(OH2)3]2, the Co would have to be Co (III), giving [Co(NH3)3(OH2)3] a +3 charge. This way two of them would have a +6 charge and even out with the (SO4)2.
You know that both amines and aqua(water) ligands have a neutral charge. Therefore, the 3+ charge on the Chromium must come from another species outside the coordination complex. Sulfate Ions have a 2- charge and knowing that there are 2 coordination complexes for every 3 sulfate ions, you know that the charge ratio between the Chromium complex and the sulfate ions is 2:3. Thus, Chromium takes on a 3+ charge in the coordination complex. Hope this helps Shirley
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