Someone just came to me with a really interesting problem. Here's the crux of
the matter:
A pyridine-imine ligand shows a chemical shift for the imine proton about 8.2ppm
and the proton ortho to the pyridine N about 8.8ppm.
When bound to PdCl2 the shifts both move downfield, as one would expect. This
is a really nice spectrum with sharp lines.
When the chlorides are replace with an electron donating ligand, the shifts do
some weird things: The imine proton moves to 9.0ppm and the ortho proton moves
to 8.8ppm. This is accompanied by a tremendous amount of line broadening.
I think this is due to ligand-ligand charge transfer. But the most surprising
effect is the fact that the H-H COSY shows a very strong coupling between sites
separated by 8 and 11 bonds.
What's going on? The complex appears pure. Are the cross peaks due to some
intraligand charge transfer, or are they through space coupling?
Mike
Dr. Michael D. Mosher
Assistant Professor / Manager NMR Lab
Department of Chemistry
University of Nebraska at Kearney
Kearney, NE 68849-1150
Ph: (308) 865-8491
mosherm@platte.unk.edu