Now a complex ion is a structure containing a metal cation that acts as a Lewis acid and covalently bonded to a ligand. Now remember a Lewis acid is an electron pair acceptor. Now a ligand is a molecule or ion that acts as a Lewis base and donates a lone pair to the metal cation.

Here in this example we have the ammonia molecule with its lone pair and we have the cadmium ion. Ammonium. Ammonium with its lone pair can donate this lone pair to the cadmium iron. Ammonia is acting as our Lewis base. Cadmium ion is accepting the electron pair, so it's acting as our Lewis acid. Now in reality, more than one of these ammonias can connect with the cadmium ion. Here we have four of them connecting to the cadmium ion.

Now you don't need to be able to figure out how many would attach, you'll learn that much later on in a later chapter. But for right now in this example we have 4 ammonium molecules connecting to the cadmium ion. Ammonia itself is neutral, has no charge, but cadmium here has A²⁺ charge. So overall the structure has A²⁺ chart. Now in the past we've talked about Lewis acid base chemistry. In the past we'd have called this an adduct because we added them together, but in this case we can also call it a complex ion. This would represent our complex ion structure.

Now, connected to the idea of complex ions is the equilibrium constant of the formation constant. Here we're going to say the formation constant is capital K_f. It is another equilibrium constant. Like other equilibrium constants, it is a ratio of product to reactive concentrations. But now they deal with complex ions. Here we're going to say like other equilibrium constants, it can be calculated by setting up an expression and ignoring the phases of solids and liquids. So just remember we have a Lewis acid Lewis space reacting together, adding together to create a complex ion. Associated with this complex ion is a K_f value, known as our formation constant.