When a metal dissolves in a solution of negative ions, like say the ethanoate (CH3COO-) in vinegar, it forms a salt. Metal salts are not molecules. In their pure form, they are ionic crystals, huge lattices of ions stacked together like marbles and held by magnetic attractions. In solution, they form complexes. A complex is a positive ion, usually a metal ion, surrounded by negative ions or neutral molecules. The ions, or the electron-rich parts of the molecules, push electrons towards the positive ion in the middle. The extra electrons make that positive ion more stable, which is why complexes form in the first place.

Something with a lot of excess charge – with too much electron density, or too little – and nowhere to store it is unstable. Almost any kind of interaction is better than nothing. A metal ion is a single nucleus without enough electrons. Unlike something big, like a dye molecule, there is nowhere else for those electrons to be supplied by. The metal is stuck with the positive charge.

So when that metal is floating around in a solution that has negative ions, or pairs of electrons that aren’t being used in bonding – even double bonds, in a pinch – the things that have spare electrons coordinate with the positive metal ion. The exact method by which this happens is too complicated to cover here, but the important point is that these electrons, the ones that are donated to the positive ion, are not forming covalent bonds. The electrons are shared between the negative thing and the ion, but the link between the two things is weak. It’s easily broken. It doesn’t take a full bond’s worth of energy to break the connection.

The connection is weak enough that the ligands (the negative things coordinated to the positive ion) can swap places with their counterparts in solution. You can use isotopically labelled ligands to track how fast this happens, and you can watch the colour change when you add a better ligand to the solution. It’s called reversible bonding, and while it may seem like an obscure branch of chemistry, that’s only because of how chemistry is taught. Reversible bonding – the formation of complexes around metal ions – is kind of important. It’s how haemoglobin transports oxygen.