[nielsgeode]

I'm currently doing my master research project and I've got a challanging problem:

I did a reaction with an organometallic (niobium) complex I synthesized with carbon monoxide. The reaction seems to be reasonably clean and it gives one main product. The question is, however, which product?

- 0.5 equivalents of acetone are formed as a side product
- It's NMR spectrum shows beside the (Cp - arene) ligand three different methyl groups which integrate as 1.5 H's
each (compared to the ligand)
- The IR spectrum shows there are NO terminal CO's
- The complex is C1 symmetrical

Concerning the integrations of the methyl groups, it has to be a dimer which can be symmetrical in two ways:

-left and right side different and front and back identical
-left and right identical and front and back different

In the first case you have two different ligands (one on each Nb) so you would expect at least 4 different signals for the Ar-H's. Because you only see three one can conclude that the dimer has the left and right side of the dimer identical.

It seems from the 1H NMR spectrum that there no other ligands present than the Cp ligand and the three methyl groups distributed over the two Nb's. If you than do electron counting on such a complex you always end up with a Nb3+ and a Nb4+ which must be paramagnetic.

There is one signal left in the carbon NMR at about 105 ppm. This is from a quaterny carbon (singlet in C-H coupled spectrum).

Is there anyone who has an idea about the structure or experiments I can do to determine the structre?

Here you can download a PDF file with the structure of the starting material and the spectra: click!