The 18 and 16 Electron Rule - Online Tutor, Practice Problems & Exam Prep

Main group elements typically follow the octet rule, aiming for 8 electrons, while transition metals adhere to the 18 and 16 electron rules for stability. The 18 electron rule, derived from the sum of 2 (s), 10 (d), and 6 (p) electrons, indicates optimal stability. Transition metals, particularly groups 8 to 11, may also stabilize with 16 electrons, as seen in palladium and nickel. Understanding these electron configurations is crucial for predicting reactivity and stability in transition metal complexes.

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.

How many NH­₃ ligands would need to be placed onto a nickel atom if we assume the resulting complex follows the 18-electron rule?

So the practice question states, how many NH3 ligands, so ammonia ligands, would need to be placed onto a nickel atom if we assume the resulting complex follows the 18-electron rule. To answer this question, we first have to figure out the number of valence electrons for our nickel atom. If we look on your periodic table, you'll see that the condensed electron configuration of nickel is argon, 4s23d8. Adding up the s and d orbital electrons means that we have 10 valence electrons for nickel. And remember, we're using the formula for electron count. The electron count formula is: Electron count = valence electrons of our transition metal - charge of our transition metal complex + x type ligands + 2 times l type ligands. In our case, since the nickel atom is neutral and our ligands are also neutral, the charge of our transition metal complex would be 0. Ammonia ligands are l type, not x type, hence the number of x type ligands is 0. We want to achieve an electron count of 18 to comply with the 18-electron rule. To reach an electron count of 8, we would need 4 ammonia ligands because it would be 2 times 4 l ligands, according to our formula. Therefore, the complex would be nickel connected to 4 NH3 ligands. Remember that transition metals usually try to follow either the 18-electron rule or the 16-electron rule to more closely resemble a noble gas configuration, whereas main group metals generally adhere to the octet rule.

Using the 18-electron rule, explain why V(CO)₆ can be easily reduced to [V(CO)₆]^–.

So using the 18 electron rule, explain why VCl6 can be easily reduced to its new complex ion form here. Alright. So to do this, we're going to figure out what the electron count is of the structure originally. We start with vanadium. If you look at your periodic table, you'll see that the electron configuration of vanadium is argon 4s2 3d3. By adding up the s and d orbital electrons, we'll see that the valence count for vanadium is 5. Minus the charge of our original structure, it has no charge so it's 0. Plus the number of x type ligands. Here our carbonyl is not an x type ligand, it's an l type ligand. So this is 0, plus 2 times the number of l type ligands which is 6. So that comes out to 17 for the electron count. Now, by gaining 1 electron, it gets a charge of minus 1. So now, its electron count becomes 5 minus a minus 1, plus 0, plus 2 times 6. So that becomes 5 plus 1 which is 6, plus another 12 is 18. Remember, when it comes to transition metals, they're trying to get to that ideal number of 18 electrons so they can more resemble a noble gas. This explains why our original structure would gladly accept an additional electron because that would take it from an electron count of 17 which is not ideal to 18 which is much more ideal. Also, remember when it comes to the valence groups from 8 to 11, some of those transition metals are happy with 16 electrons. Vanadium doesn't fall within the 8 to 11 valence groups, so it wants to get to 18 electrons. So, it would much rather gain an electron in that case, an electron count will become 16, which would not be ideal for vanadium. And remember the valence groups of 8 to 11, the two transition metals that are okay with having 16 for their electron count are predominantly nickel and palladium. Again, vanadium doesn't fall within that group, so it wants to get to 18. So its best case would be to gain one electron like we have here. So, just remember, we're going to be applying this 18 and 16 electron rule to discuss how transition metals would rather gain electrons or lose electrons to become more stable within our organic reactions.

Here’s what students ask on this topic:

What is the 18 electron rule in transition metal chemistry?

The 18 electron rule in transition metal chemistry states that the most stable transition metal complexes have a total of 18 valence electrons. This rule is derived from the sum of the maximum number of electrons that can occupy the s, p, and d orbitals: 2 (s) + 6 (p) + 10 (d) = 18 electrons. Achieving this electron configuration allows the transition metal to attain a noble gas-like stability. This rule is particularly useful for predicting the reactivity and stability of transition metal complexes.

Why do some transition metals follow the 16 electron rule?

Some transition metals, particularly those in groups 8 to 11, can achieve stability with 16 valence electrons, a phenomenon known as the 16 electron rule. This occurs because these metals can form stable complexes without fully occupying all available orbitals. Palladium and nickel are common examples of metals that often follow the 16 electron rule. This rule is important in understanding the reactivity and catalytic behavior of these metals in various chemical reactions.

How is the 18 electron rule related to the octet rule?

The 18 electron rule for transition metals is analogous to the octet rule for main group elements. While the octet rule states that main group elements are most stable with 8 valence electrons, the 18 electron rule indicates that transition metals are most stable with 18 valence electrons. Both rules aim to describe the most stable electron configurations, with the octet rule focusing on s and p orbitals, and the 18 electron rule including s, p, and d orbitals.

What are the exceptions to the 18 electron rule?

Exceptions to the 18 electron rule often occur with transition metals in groups 8 to 11, where complexes can be stable with 16 valence electrons. This is known as the 16 electron rule. Palladium and nickel are notable examples of metals that frequently exhibit this behavior. These exceptions are important for understanding the reactivity and catalytic properties of certain transition metal complexes, especially in organic chemistry reactions.

How do you calculate the total valence electrons in a transition metal complex?

To calculate the total valence electrons in a transition metal complex, sum the valence electrons of the metal and the electrons donated by the ligands. For the metal, consider its group number and any oxidation state. For the ligands, count the electrons they donate (e.g., CO donates 2 electrons). Add these values to get the total valence electron count. This helps determine if the complex follows the 18 or 16 electron rule, indicating its stability.