Electron Counting - Online Tutor, Practice Problems & Exam Prep

Electron counting is essential in main group chemistry, particularly for understanding the octet rule and reactivity. Elements with fewer than 8 electrons can act as Lewis acids by accepting electron pairs. For transition metals, the electron count is determined using the equation: Electron Count=Valence of Metal-q+x+2⁢l. This approach helps predict stability and reactivity in transition metal complexes.

Electron Counting

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 4s²3d⁸. 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 PPh₃, 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.

What is the electron count of the complex ion [Cr(NH­_3­)₄Cl­₂]⁺?

Alright. So here we have to figure out the electron count of the complex. First, we start out with our transition metal which is chromium. Now remember, chromium has a different or exception to our electron configuration. Its real configuration will be argon s1d5. So the number of s and d orbital electrons comes out to 6. So that's the valence electron count for chromium minus the charge of our complex which is plus 1. So that's minus 1. Plus our X type ligands, Cl. It's an X type ligand because it exists as Cl−. There are 2 of them. Plus 2 times the number of L type ligands which would be your ammonia molecules here. So that's 2 times 4. So that comes out to 6−1+2+8. Crunch those all into your calculator or you can do it on paper. It gives us an electron count of 15. So remember, electron counts can be pretty simple. Just treat the transition metal, regardless of its charge. Observe it as though it's neutral. From that, calculate its number of valence electrons. Once you do that, look at the charge of your complex, whether it has a charge or not. Then be able to distinguish between X and L type ligands. Plug all that in and you'll get your electron count at the end. Remember, this is just the first step to help us understand how the reactivity and stability of transition metals themselves tie into later organic chemistry-based reactions.

Electron Counting Exercise 1

So here it says what is the electron count of the complex ion? We have cobalt connected to 6 cyanide ligands and the overall charge is 3 minus. Alright. So cobalt, remember for the transition metal, for its valence electrons when it comes to these complex ions just consider it neutral to get the correct electron count. Cobalt, if you look on the periodic table, it would be argon \(4s^2 3d^7\). If we add up the number of s and d orbital electrons, the valence count for cobalt would be 9. Then minus the charge of the complex. It's negative 3. So that's minus a minus 3. Plus the number of x type ligands. Cyanide ions are negative, so they're an x type ligand and there are 6 of them. And then we don't have any l type ligand so that's 2 times 0. So minus of a minus is positive. So 9 plus 3, plus 6 equals 18 electrons for electron count. It's as simple as that. Remember to use the formula and you'll be able to figure out the electron count for any transition metal complex. We have to consider the number of valence electrons for the transition metal as though it's neutral. Then remember we have the charge of the complex overall and just figure out which ligands are x type, which ones are l type, input them into the formula, and get your final answer. Now that you've seen 2 examples so far of how to do electron count, let's see if you can do the final one left here as practice. Attempt it on your own. Once you do, come back and see if your answer matches up with mine.

Here’s what students ask on this topic:

What is electron counting in transition metal complexes?

Electron counting in transition metal complexes involves determining the total number of valence electrons around the metal center. This is crucial for predicting the stability and reactivity of the complex. The electron count is calculated using the formula:

$\mathrm{Electron} \mathrm{Count}=\mathrm{Valence} \mathrm{of} \mathrm{Metal} -q+x+2l$

Here, the valence of the metal is the sum of its s and d orbital electrons in its neutral state. 'q' is the charge of the complex, 'x' represents the number of X-type ligands (donating 1 electron each), and 'l' represents the number of L-type ligands (donating 2 electrons each).

How do you determine the electron count for a transition metal complex?

To determine the electron count for a transition metal complex, follow these steps:

1. Identify the transition metal and its neutral valence electron configuration (sum of s and d orbital electrons).
2. Determine the charge of the complex (q).
3. Count the number of X-type ligands (each donating 1 electron).
4. Count the number of L-type ligands (each donating 2 electrons).

Use the formula:

$\mathrm{Electron} \mathrm{Count}=\mathrm{Valence} \mathrm{of} \mathrm{Metal} -q+x+2l$

For example, for a nickel complex with 10 valence electrons, no charge, 2 X-type ligands, and 2 L-type ligands, the electron count would be:

$10-0+2+4=16$

What is the octet rule in main group chemistry?

The octet rule in main group chemistry states that atoms tend to form bonds until they are surrounded by eight valence electrons, achieving a stable electron configuration similar to noble gases. This rule helps predict the reactivity and bonding behavior of elements. For example, carbon in formaldehyde (CH₂O) forms four bonds, fulfilling its octet. Nitrogen in nitrogen trichloride (NCl₃) has three bonds and one lone pair, also satisfying the octet rule. Oxygen in water (H₂O) has two bonds and two lone pairs, completing its octet.

Why is electron counting important in transition metal chemistry?

Electron counting is crucial in transition metal chemistry because it helps predict the stability, reactivity, and electronic structure of metal complexes. By knowing the electron count, chemists can determine whether a complex is likely to undergo certain reactions or remain stable. This understanding is essential for designing catalysts and understanding the mechanisms of transition metal-catalyzed reactions. The electron count provides insights into the bonding and electronic properties of the metal center, guiding the development of new materials and chemical processes.

How do X-type and L-type ligands differ in electron counting?

X-type and L-type ligands differ in the number of electrons they donate to the metal center. X-type ligands donate one electron each, while L-type ligands donate a lone pair, which consists of two electrons. In the electron counting formula:

$\mathrm{Electron} \mathrm{Count}=\mathrm{Valence} \mathrm{of} \mathrm{Metal} -q+x+2l$

'x' represents the number of X-type ligands, and 'l' represents the number of L-type ligands. For example, in a complex with 2 X-type ligands and 2 L-type ligands, the contribution to the electron count would be:

$2+22=6$