Oxidation Number in Naming

[alexandrad 3J]

I'm confused as to why the oxidation of certain compounds appears and sometimes it does not. For example, for tetraamminediaquacobalt(III) bromide, the number within the parenthesis is apparent in the chemical formula as shown respectively: [Co(NH3)4(h20)2]Br3. However, for sodium bisoxalato(diaqua)ferrate(III) it does not: Na [Fe(OH)2(C2O4)2]. Why is this?

[KaylaH]

I'm a little confused by your question. Are you asking why is the oxidation state in the second example is not "as obvious" as the first? If so, I guess my response to that is that you have to take into consideration the charges of the ligands in order to find the oxidation state of the TM.

In the second example, the coordination complex is an anion so it must have an overall charge of -1. (Because it is combined to Na which is +1). The (ox) has a charge of -2, and since there are 2, it has an overall charge of -4. So, in order to achieve an overall charge of -1, Fe must be +3.