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Number 17.31a asks for the formula of potassium hexacyanidochromate(III). From my understanding, the roman numerals at the end of the name implied that the chromate ion had an oxidation state of 3+. This would mean that the [Cr(CN)6] part of the compound would have a net positive charge, and if it were to bond with potassium, the potassium would have to have a negative charge. I know that potassium is almost always a cation; however, I thought chromium was the same way, and one of them has to be negative. Are we just supposed to assume that the roman numerals are indicative of a negative charge on the Cr ion because potassium has such a low ionization energy? Is it common for transition metals to have negative charges in coordination compounds?
Actually, the fact that it is potassium hexacyanochromate(III) means that the potassium has a positive charge (+1). The fact that Chromium has a 3+ oxidation state does not make the whole hexacyanochromate(III) complex have a net positive charge. Also, the fact that the complex includes "chromate" rather than "chromium" indicates the complex [Cr(CN)6] has a net negative charge. You can prove this by looking at a couple things: since Cr is 3+, as you said, and CN usually has a -1 charge and there are 6 CN ligands, then the overall charge of the complex is (+3)(1)+(-1)(6)=-3. This means that the coordination compound would be K3[Cr(CN)6].
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