It all started with a question that one of my friends asked a few weeks ago. He came to me and asked what kind of ligand pyridine was. Without hesitating I said that it is a "strong field" ligand. Suddenly I was bothered by my answer. Not that it was wrong, but I realized that bipyridine was a stronger field ligand. Why was that? I looked at spectrochemical series maybe hundred times and I have never wondered why this was the case. Moreover, (although I know the reason for the general trend in the series) I was never curious why the series followed the order of pyridine<ammonia<ethylenediamine<bipyridine<phenathroline .
image: spectrochemical series (as you move right, you go to the stronger field ligands)
In order to find an answer (as expected), I googled things! Looks like someone else also asked a similar question and a discussion took place on Researchgate website here.Well, you can read the answers to the question but I am not satisfied. Those answers are not THE ONE I am looking for. I feel like this trend should be explained in a better way. Since I was busy with my final lab reports, assignments, finals, personal life and the surprises of life etc., I did not have enough time to read and find out an answer. Today, I tried to look for literature on spectrochemical series. Thanks to chemistry gods, I found this one:
"The Position of 2,2'-Bipyridine and 1,10-Phenanthroline in the Spectrochemical Series"
It is not what I was looking for. But, it is very very very helpful. Better news (for some of you) is that the pdf file is free to read here. It is a really interesting article even with the "dedication" part. The authors from Denmark dedicated the paper to Prof. K. A. Jensen for his 70th birthday. I know there is a term for these papers, but I forgot.
Anyway, although the paper was published in 1977, I think it is awesome. This is exactly one of the main reasons I LOVE inorganic chemistry. There are theories, there are not fully investigated complexes and trends. There is a lot of thinking, experimenting and discussing. People come up with ideas, theories and you can challenge them if you work hard and carefully. And some luck ? Sure.
I quickly read the paper and I will really spend time on it tomorrow. So, I might write another post after reading it or I can wait until I find more answers. We'll see. In summary, the authors prepared several cobalt and chromium bipy and phen complexes and studied/compared their spectra.
For cobalt(III) complexes the series;
and for chromium(III) complexes the series is as follows:
Please don't hesitate to suggest papers or answers for the trend in the series. I believe I will find a satisfying answer since these ligands are among the most used and studied ligands in inorganic chemistry.