The later transition metals are smaller than the earlier ones - on crossing the 3d transition series, the nuclear charge (and atomic number) increase, but the screening due to filled electronic shells stay the same. Most often they do, but the coordination number of the metal is largely dictated by how many donor atoms can fit around the central metal. I thought that transition metals generally form octahedral complexes? Water produces larger splittings between the energy levels than chloride (H 2O is a ‘stronger-field’ ligand than Cl) so the energy differences between different electronic energy levels are smaller in complexes with lots of chlorides as ligands, so this has a significant effect on the colour. The colour of transition metal complexes is due to electronic transitions between different d-orbitals. It can be explained using ligand-field theory. Quite a few tetrahedrally coordinated Co(II) complexes, like those containing the 2- ions, are blue in colour, so at one time people believed it was because anhydrous CoCl 2 contained tetrahedrally coordinated Co(II). Why does the colour change between the different forms of cobalt chloride? It looks as if 2+ and + ions are the main species present in solutions of CoCl 2. It shows that it is the balance of small things that can tip the balance and determine a structure. In the tetrahydrate CoCl 2.4H 2O, it appears that hydrogen bonds between chlorides and the hydrogen atoms in water molecules in the next CoCl 2(H 2O) 4 molecule along make the cis structure more favourable. Yes, normally the bulky chloride ligands stay as far apart as possible in the coordination sphere. If you carry out the crystallisation of cobalt chloride around 50✬, you get the tetrahydrate CoCl 2.4H 2O, which surprisingly contains cis- molecules. Each cobalt is surrounded by six chlorides, each chloride being shared between three cobalts.Ī 3D version of this can be seen at ChemTube3d.Ī smaller section of a Cobalt(II)-chloride-layer.ĬoCl 2.2H 2O has a chain structure in which each cobalt atom is bound to 4 bridging chlorides and to two water molecules.ĬoCl 2.6H 2O contains trans- molecules, with two molecules of ‘water of crystallisation’ in the lattice, so you can write it. How does the structure change as water molecules are added?Īnhydrous CoCl 2 has the same structure as cadmium chloride, CdCl 2. Silica gel is sometimes supplied containing some cobalt chloride, which indicates the water content – blue when it is dry, pale pink when the air round it is damp. When you heat the gel, it loses the water so you can reuse it. Yes, they are put in with various consumer products, like electrical goods, shoes and handbags, to absorb water, because silica gel can absorb up to about 40% of its weight in water. Silica gel? You mean the stuff that comes in little bags with a warning not to eat them? It can be used as a humidity indicator in weather-measuring instruments, as well as in self-indicating silica gel. You can get an indicator paper – like litmus paper but containing cobalt chloride - which changes colour from blue to pink in the presence of water. This can again be reversed by adding water. If you warm pink CoCl 2.6H 2O gently – up to 150☌ or so - it will gradually lose water forming the violet CoCl 2.2H 2O, then blue anhydrous CoCl 2. You can reverse these changes with solid cobalt chloride crystals. First the purple dihydrate, CoCl 2.2H 2O, forms and then at higher humidity the pink hexahydrate CoCl 2.6H 2O is the product. Anhydrous cobalt chloride, CoCl 2, is blue in colour. It is an unusual material in the way it changes colour, particularly in the presence of water. Why be interested in cobalt chloride anyway? You can also obtain isolated CoCl 2 molecules in matrices of argon and nitrogen at low temperatures too (the unreactive Ar atoms and N 2 molecules come between CoCl 2 molecules, ‘spacing them out’). Electron diffraction measurements show that it is a linear molecule with Co-Cl distance of 2.113 Å. True, but this picture shows the structure of an isolated CoCl 2 molecule in the gas phase at around 1000 K. Why? Everyone knows that a metal and a non-metal form ionic bonds and have giant structures. COBALT CHLORIDE - Molecule of the Month June 2016 - JSMol versionĬOBALT CHLORIDE A drug used to dope racehorses that's also a water indicator.
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