Reaction of (RhCp*Cl2)2 with butadiene
The precatalyst system [RhCp*Cl2]2 was developed by Peter Maitlis and his group at the University of Sheffield in the 1970's and 80's. It has been used as a hydrogenation catalyst under a number of conditions, and is readily commercially available. Under the right conditions, ethanol can act as a suitable transfer hydrogenation source (not quite as good as 2-propanol, but better than methanol), but in this case, in the presence of a suitable base (sodium carbonate), the rhodium dimer will react to give a stable product.

$$\begin{array}{|c|c|c|}
\hline
\textbf{label} &amp; \delta \textbf{ / ppm} &amp; \textbf{Relative intensity} &amp; \textbf{Multiplicity} \\ \hline
\textbf{A} &amp; 1.65 &amp; 3 &amp; \text{d, } J = 6.2\ \mathrm{Hz} \\ \hline
\textbf{B} &amp; 1.73 &amp; 15 &amp; \text{s} \\ \hline
\textbf{C} &amp; 2.88 &amp; 1 &amp; \text{dd, } J = 11.0,1.5\ \mathrm{Hz} \\ \hline
\textbf{D} &amp; 3.28 &amp; 1 &amp; \text{dd, } J = 6.6,1.5\ \mathrm{Hz} \\ \hline
\textbf{E} &amp; 3.58 &amp; 1 &amp; \text{dq, } J = 7.0,6.2\ \mathrm{Hz} \\ \hline
\textbf{F} &amp; 3.96 &amp; 1 &amp; \text{ddd, } J = 11.0,7.0,6.6\ \mathrm{Hz} \\ \hline
\end{array}$$

From the NMR, it is possible to deduce what structure is likely, if we construct a valid spin system from the available data.


signal B: &delta;1.73 integral of 15 : we still have a Cp*
signal A: &delta;1.65 integral of 3 : looks a lot like we have a methyl group. It has a doublet coupling of 6.2Hz, which corresponds to signal E at &delta;3.58. 
signal E: &delta;3.58 is single 1H which appears as a dq. We can account for the q coupling; it is next to a methyl group, which leaves the doublet coupling of 7.0Hz, which corresponds to signal F at &delta;3.96
signal F: &delta;3.96 is a single 1H with additional coupling to 2 other single 1H nuclei; one of 11.0Hz (signal C), which is pseudo-trans, and the other at 6.6Hz (pseudo-cis, signal D). This two signals have a mutual coupling of 1.5Hz, typical of a pseudo-gem couping on a double bond.


So that gives a spin system skeleton that looks like:



which, is able to form a stable &pi; complex.
Homework spoiler alert:


   


and a useful reference from many years ago....


   K. Moseley,   J. W. Kang and    P. M. Maitlis J. Chem. Soc. A, 1970, 2875-2883. DOI: 10.1039/J19700002875