Coordination Compounds

I had an acquaintance with this question of coordination compounds which I find difficult to solve:

We have a coordination compound of ML3_3 type structure where the ligand L is .

Now find the total number of stereoisomers.

Please help me through this problem!

#Chemistry

Note by Sai Krishna Attaluri
4 years ago

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Comments

The ligand LL stated above is a methyl substituted derivative of ethylenediamine ( NHX2CHX2CHX2NHX2\ce {NH2 - CH2 - CH2 - NH2} or en\ce {en} ). This ligand is a bidentate chelating ligand where both the nitrogen atoms donate their electrons to the metal ion. Thus the ligand has a coordination number 66 and not 33 and it looks like a propeller, like this example

A complex of \(\ce {[Fe (Me-en)3]}^{3+}\) A complex of [Fe (Meen)X3]3+\ce {[Fe (Me-en)3]}^{3+}

As you can see this has three chiral centers and no restricted rotation sites. So by using the three formulas of finding the number of stereoisomers when number of chiral centers is odd as listed below

No. of enantiomers =2n12(n1)/2No. of meso compounds =2(n1)/2No. of optical stereoisomers =2n1\begin{aligned} & \text{No. of enantiomers } = 2^{n-1} - 2^{{(n-1)}/{2}} \\ & \text{No. of meso compounds } = 2^{{(n-1)}/{2}} \\ & \text{No. of optical stereoisomers } = 2^{n-1} \end{aligned}

We respectively get

No. of enantiomers =2No. of meso compounds =2No. of optical stereoisomers =4\begin{aligned} & \text{No. of enantiomers } = 2 \\ & \text{No. of meso compounds } = 2 \\ & \text{No. of optical stereoisomers } = 4 \end{aligned}

Tapas Mazumdar - 4 years ago

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That's true but I faced problem in counting the cases of geometrical isomers in each case above.

Sai Krishna Attaluri - 4 years ago

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Is my answer correct? Do you know what the answer is? I didn't make a count btw, just went with the formula.

Tapas Mazumdar - 4 years ago

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@Tapas Mazumdar The answer is 24 stereoisomers. Your answer must be correct regarding optical isomers as I had tried it (in another way) and got the same number of optical isomers but I am not certain.

Sai Krishna Attaluri - 4 years ago

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@Sai Krishna Attaluri Yes, I got the answer. The number of optical isomers is 44 along with that if you also account the position of methyl group on each ligand, you can have a possible of 66 configurations wrt position of methyl group, thus 4×6=244 \times 6 = 24. But this is the total number of all possible isomers.

Tapas Mazumdar - 4 years ago

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@Tapas Mazumdar I will think on this and get back.

Sai Krishna Attaluri - 4 years ago
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