The 18 and 16 Electron Rule - Video Tutorials & Practice Problems

The 18 and 16 Electron Rule

In this video, we're going to take a look at the 18 and 16 electron rule. Now, remember, main group elements want to follow the octet rule because that gives them the ideal number of electrons just like a noble gas. Main group elements, remember, are groups 1A to 8A. And if we look there are 8 slots. So 1, 2, 3, 4, 5, 6, 7, 8. That's what explains why 8 is the ideal number in terms of the octet rule. Exceptions to this are hydrogen and helium. Helium only has a configuration like helium. When it comes to transition metals though, in transition metal chemistry, we use the 18/16 electron rule as an indicator for the reactivity of a transition metal. We're going to say the most stable transition metal complexes in several cases have electron counts of 18.

Okay, 18 electrons. This trend, because we're dealing with 18 electrons, is called the 18 electron rule. How do we come up with this number of 18? Well, the most stable number of electrons represent the number of total s, p, and d electrons together. Remember, your s orbitals have 2, d orbitals have 10 electrons max they can hold, and p orbitals have 6. So when we add those up together, 2 plus 10 plus 6, that gives us 18. That's where this 18 electron rule comes from. And remember, when it comes to these transition metals, they have the number of valence electrons based on the number of s and d orbital electrons. So from 3, all the way up to 12. Now, there are exceptions. 18 for a transition metal is great for it to get to a noble gas configuration by incorporating its z orbitals, but there are exceptions to this. We're going to say exceptions often happen with transition metals from 8 to 11 valence electron groups. So, we're talking about the transition metals within this sector and below.

Now, we're going to say the tendency of these metals to be happy with 16 electrons is called the 16 electron rule. For a lot of the transition metal catalyzed reactions that we're going to see in organic chemistry, the 2 most common transition metals that do this 16 electron rule are palladium and nickel. So they could follow the 16 electron rule. Of course, all transition metals would love to get 18 electrons because that would be the best configuration to become more like a noble gas. But, palladium and nickel especially are also okay with 16. We take this into consideration when thinking about transition metals. Whereas main group elements are trying to get 8 for the octet rule, transition metals in terms of stability are always aiming for 18 and in a few cases where 16 is okay. Now, now that we've talked about the 18 and 16 electron rule for transition metals, we'll move on to examples in terms of electron counts. So just remember, transition metals ideally want 18 electrons whereas main group metals we're used to seeing them ideally wanting 8 electrons.

The 18 and 16 Electron Rule Example 1

So here the example states, "What is the electron count of the neutral transition metal complex of Br₂Pd(CO)₂?" Alright. So first we have palladium as our transition metal. We have to give its electron configuration to determine the number of valence electrons it possesses. So remember, palladium has a unique electron configuration. It's just Krypton 4d¹⁰. It has no s orbital electrons. Those are promoted in order to completely fill up our d orbitals. So we have 10 d orbital electrons. So the valence electron count for palladium is 10 minus the charge of our complex. Here it has no charge; we're told it's neutral, so it's 0. Plus the number of X ligands, which in this case would be the 2 bromines, plus 2 times our L ligands. Here, we have carbonyl as our 2 L ligands. So that's 2 times 2. So here this comes out to 10 plus 2 plus 4. So, in this case, palladium is following a 16 electron rule.

Now that we've seen this one, we'll move on in the next video to the second example. This one may not be what you think it is. Think of what kind of ligand is our en. What is that abbreviation for? That's going to play a role in terms of calculating our electron count for this particular complex. So come back and see how I approach example 2.

The 18 and 16 Electron Rule Example 2

So this one is not exactly what you may think it is. Here it says what is the electron count of the neutral transition metal complex of palladium and ethylenediamine. Remember, ethylenediamine is NH₂CH₂CH₂NH₂. Here we have 2 locations for a lone pair. This is a bidentate ligand, so we're going to figure out what our number of valence electrons are for palladium. It's still 10; that's not going to change. Minus the charge, which is 0. Ethylenediamine is not an x-type ligand; it's an l-type ligand. So x here is 0 plus 2 times the number of l-type ligands. Now, for the formula, 2 times l-type ligands is based on the number of lone pairs or electron donating sites on the l-type ligand. But this is a bidentate ligand. It has 2 locations that can donate electrons. So, basically, we have 2 bidentate ligands, which is equivalent to 4 l-type ligands. Because there are 4 sites total, through which we can have the donation of electrons through lone pairs. So this is actually 2 times 4. So that's 18 for the electron count. So again, going back to the original equation for electron count, in the part of the equation, it's 2 times l-type ligand, based on the number of electron-donating sites. So for monodentate, it would just still stay as 1. But for bidentate, it'd be double that. So that's why we have 18 as our electron count here. Just keep that in mind when figuring out the electron count for any transition metal complex.