1

concept

## Electron Counting

7m

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in this video, we're gonna take a look at electron county. 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 like a noble gas, it'll be more reactive. A lot of reactions in gen kim can be explained if we observe them as a lewis acid based reaction. Now, if an element possess less than eight electrons around it, then it would accept an electron pair in this case is 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 three compounds, we can see that through a combination of sigma bonds, pi bonds and or lone pairs. The central element is fulfilling the octet rule. For the first compound we have formaldehyde. Carbon here is making four bonds. And through all the electrons in these four bonds, carbon fulfills its octet rule of having eight electrons around it. Next we have nitrogen tri chloride where it has a lone pair and then three single bonds. And it to fulfills the octet rule. And then finally water, we have oxygen with its two lone pairs and two single bonds. So in all three cases we have different combinations of sigma bonds, pi bonds and or lone pairs. And all the central elements are following the octet rule when it comes to transition metals though they have d orbital's which allows them to expand beyond eight electrons around them. So we'll have to figure out new rules to talk about their stability and or reactivity. But before we can talk about those new rules, we first have to master Elektron County. 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 will 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 transition metal complex, we employ the following equation. Now the equation for electron count is equal to valence of metal M minus Q sub M plus X type legans plus two times L type Liggins. If we break this equation down into its components, we're gonna say valence of metal M metal M is just another way of talking about a transition metal. So the valence electrons of a transition metal M remember are equal to your s orbital electrons and your d orbital electrons together. If we're looking at nitrogen, It's condensed electron configuration is Argon four, S 2, three D 8. By adding up the S and D orbital electrons, We see that Nickel has 10 valence electrons. Qm just represents the charge of the transition metal complex for this one here we have zinc connected to four water Liggins and the overall charges two plus. So that is what Q. M. Is equal to here. We have another transmittal transition metal complex, this one has no charge on it. So Q sub um is equal to zero. Now the beauty about this equation, before we go on to X type Liggins is that when it comes to the valence of our transition metal M. This is just when we're looking at when we're considering the metal in its neutral form. So regardless of what the charge is of the transition metal in the complex, we don't worry about that. All we're gonna do is base the number of valence electrons on the neutral form of that transition metal. Okay, so it doesn't matter if it's nickel two plus or nickel, whatever 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 Liggins and L type Liggins. And notice that for X type Liggins, it's the one here. But for L type Liggins is a to that's because when it comes to X type Liggins, they donate one electron to the metal cat ion of the complex molecule or ion chlorine is in group seven A. So they each have seven valence electrons here. And one of those electrons is being used to make this bond here with the nickel. So X type Liggins tend to donate one electron to the metal Cat eye on L type Liggins though they donate a lone pair which is composed of two electrons. So here we have PPH three. Remember P. H. Stands for benzene. These electrons in this bond come from the phosphorus because remember when we're talking about the different types of Liggins, phosphorus has a lone pair there. Now we have our structure here and 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 what charge it is. Is 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 it charges zero plus. Remember halogen like chlorine are X type Liggins and there's two of them Plus two times these are L type Liggins. So two times 2. So we have 10 minus zero plus two plus four. So the electronic count here would be 16. And that's all we have to do in terms of electron count. So remember when it comes to main group elements, we have the octet rule, transition metals have D orbital's which allow them to expand beyond the octet rule. Our first step in understanding their eventual rules is to first learn how to do electron county. When it comes to this formula, it doesn't matter what the charge of the transition metal is. Look at it as though it is neutral and from its neutral form. Look at its condensed electron configuration count up the S and D orbital electrons. Then you look at Q. M, which is the overall charge of the complex, plus the number of X type Liggins, plus two times the L type Liggins. Remember these components, and you'll always be able to calculate the electron count for any transition metal complex.

2

Problem

What is the electron count of the complex ion [Cr(NH_{3})_{4}Cl_{2}]^{+}?

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3

example

## Electron Counting Exercise 1

1m

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So here it says, what is the electron count of the complex ion? We have cobalt connected to six cyanide Liggins and the overall charge is three 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 four us to three D 7. If we add up the number of S&D orbital electrons, the valence count for cobalt would be nine then minus the charge of the complex. It's negative three. So that's minus a minus three. Plus the number of X type Liggins cyanide ions are negative. So they're an X type Ligon and there are six of them. And then we don't have any type Ligon. So that's two times zero. So minus of a minus is positive. So nine plus 3 plus six equals 18 electrons for electronic count. And it's as simple as that. Remember to use the formula and you'll be able to count 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 then just figure out which Liggins R X. Type which ones are all types and put them into the formula And get your final answer. Now that you've seen two examples so far of how to do electron count. Let's see if you can do the final one left here as practice attempted on your own. Once you do come back and see, does your answer match up with mine.