In this video, we're going to take a look at electron counting. In main group chemistry, we use the octet rule as an indicator of reactivity. So basically, when an element doesn't have the optimal or ideal number of electrons, it acts as a Lewis acid-based reaction. Now, if an element possesses less than 8 electrons around it, then it would accept an electron pair. In this case, it's acting like a Lewis acid. And by accepting that electron pair, it becomes closer to the ideal number like a noble gas.

Here if we take a look at these 3 compounds, we can see that through a combination of sigma bonds, pi bonds, and lone pairs, the central element is fulfilling the octet rule. For the first compound, we have formaldehyde. Carbon here is making 4 bonds. And through all the electrons in these 4 bonds, carbon fulfills its octet rule of having 8 electrons around it. Next, we have nitrogen trichloride where it has a lone pair and then 3 single bonds and it too fulfills the octet rule. And then finally, water. We have oxygen with its 2 lone pairs and 2 single bonds. So in all three cases, we have different combinations of sigma bonds, pi bonds, and lone pairs and all the central elements are following the octet rule.

But before we can talk about those new rules, we first have to master electron counting. That's because electron count is also important in our understanding of the mechanistic basis of transition metal catalyzed reactions. Once we learn how to count the electrons correctly in a transition metal complex, we'll know how many electrons a transition metal would need in order to become more stable like a noble gas.

Now, to determine the electron count for a transition metal complex, we employ the following equation: ElectronCount = valence of metal M - Qm + Xtypeligands + 2timesLtypeligands

If we break this equation down into its components, we're going to say valence of metal M. Metal M is just another way to talk about a transition metal. So the valence electrons of a transition metal M are the total of the s orbital electrons and the d orbital electrons together. If we're looking at nickel, its condensed electron configuration is argon 4s^{2}3d^{8}. By adding up the s and d orbital electrons, we see that nickel has 10 valence electrons. Q_{m} just represents the charge of the transition metal complex. For this one here, we have zinc connected to 4 water ligands and the overall charge is 2+. So that is what Q_{m} is equal to.

When it comes to valence of our transition metal M, regardless of what the charge is of the transition metal in the complex, we don't worry about that. All we're going to do is base the number of valence electrons on the neutral form of that transition metal. It doesn't matter if it's nickel 2+ or any other charge. We look at it and consider it only in its neutral form. So the number here will always be 10 for nickel.

Now we have x type ligands and L type ligands. Notice that for x type ligands, it's a 1 here, but for L type ligands, it's a 2. That's because when it comes to L type ligands, they donate a lone pair which is composed of 2 electrons. So here we have PPh_{3}, where these electrons in this bond come from the phosphorus.

Based on the formula for electron count, let's see if we can figure out what the answer would be. We figured out the valence electron number for nickel regardless of the charge it has. It's 10 here. Minus the overall charge of the complex, here it's not in brackets and doesn't have a charge on the outside, so its charge is 0. Plus, remember halogens like chlorine are x type ligands and there are 2 of them. Plus 2 times, these are L type ligands, so 2 times 2. So the electron count here would be 16.

Remember these components, and you'll always be able to calculate the electron count for any transition metal complex.