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Hi! A simple way to determine coordination number is just to see how many ligand bonds are formed with the transition metal. It's not exactly the number of atoms that it's bonded with because some of them can be polydentate, so just looking at the number of bonds formed should give you the coordination number.
America Alvarado wrote:How do you calculate the coordination number? I'm really lost.
This is long, but I promise this could help!
I know exactly how you feel, and I just found out how to do the coordination number with the help of the step-up sessions, so let me give you a few examples, and I hope that will help! I know that the new terminology can be daunting, but you got this!
So as everyone has been saying, the coordination number correlated with the ligands, which would be the compounds/molecules that are able to donate an extra lone pair to make a bond with a metal ion. But just hearing the definition doesn't tell all right? So then the examples I'm about to give should help you with getting the feel of identifying ligands.
I grabbed this from the homework, but here are two compounds that I will be walking you through: [M(NH3)4Cl2] and K3[CoF6]
For [M(NH3)4Cl2], you want to focus on the molecules you can see, and ignore the metal Ion M. So, you can see that there is (NH3)4 and Cl2. Looking more carefully, you notice that NH3 is one ligand (ammine is its name) and Cl, which would be chloro (chlorine). ammine and chlorine are both monodentate, meaning that they can only bond once to the metal ion because of their lone pairs. NH3 has one lone pair that will make one bond while chlorine has 3 lone pairs, but 2 lone pairs "cancel" each other out, so they only have 1 available lone pair to bond. That being said, that means that each molecule of NH3 will add 1 to the coordination number, and so will each atom of Chlorine. Since there are 4 molecules of NH3, they have a total of 4 for their coordination number, and since there are 2 atoms of chlorine, they bring a total of 2 for their coordination number. Add them together, and your coordination number is 6. Most ligands are monodentate so you normally count how many ligands there are and it should match your coordination number, but some are polydentate, and they are listed in the textbook.
For K3[CoF6], again, you only look at what is inside the brackets, so ignore the 3 potassiums, they do not matter to your coordination number. Great, now you focus on the amount of Fs (fluorines) there are since cobalt is the metal ion. Each Fluorine, like chlorine, is a ligand that can only bond once to a metal ion, so their coordination number would be 1 per Fluorine. Based on that, then your coordination number for K3[CoF6] should be 6 since there are 6 fluorines.
If you can see, the ligands are every single molecule/atom that is within the brackets (normally called the coordination sphere), excluding the metal ion.
I hope that helps!
Nicole Huang 1I wrote:Hi! A simple way to determine coordination number is just to see how many ligand bonds are formed with the transition metal. It's not exactly the number of atoms that it's bonded with because some of them can be polydentate, so just looking at the number of bonds formed should give you the coordination number.
Thank you Nicole, using this as a reference extremely helpful.
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