Question

What is the oxidation no if Hg

Answer 1

Ab initio’, that is, without any knowledge of descriptive chemistry, this is tricky. But… mercury has oxidation states either +1 or +2. Cobalt has oxidation states either +2 or +3 (there are a few other, much rarer, ones, but we can ignore these, at least for a first attempt). Now, thiocyanate is a mono-anion, (NCS)-. So, then, it is possible that it could be Hg(I)[Co(III)(NCS)4] or it could be Hg(II)[Co(II)(NCS)4]; both would ‘balance’. In fact, Co(III), for various reasons I won’t go into here, massively prefers to have octahedral geometry, i.e. to have six ligands, whereas here, it clearly only has four. This immediately suggests it’s Hg(II)[Co(II)(NCS)4]. And in fact, this is correct. It’s a typical dark blue tetrahedral complex of Co(II). And if you were to measure its magnetic moment, you would find that it has three unpaired electrons/Co, which confirms it’s Co(II). Historical note: this compound used to be used as a standard for calibrating Gouy balances, which are used for measuring the magnetic moments of solids.