Structure of Compounds with 4e Ligands

In the compound [Mn(C4H6)2 (CO)], the hydrogen atoms bonded to the middle 

The C-C distances in cyclobutadiene complexes are the same generally the substituents are located away from the metal with respect to the plane of the ligand. Exceptions are encountered in some complexes possessing substituents which force their tilting toward the metal. The dinuclear compound which is prepared by reaction (8.3) may 

The proton nuclear magnetic resonance spectrum of free /raw -butadiene consists of two groups of signals. The signal located at lower fields (t ca 3.8) corresponds to protons connected to and atoms, while the doubly intense signal at t = 4.9 corresponds to protons at and C.  

The spectrum of the iron complex [Fe(C4H6)(CO)3] is of A2M2X2 type. Owing to coordination, the bands are shifted in the direction of stronger fields. The internal protons give a signal at t = 4.7, the syn protons at t = 8.3, and the anti hydrogens at T = 9.78. Therefore, the spectra of butadiene complexes are similar to those of allyl compounds. 

3-diene compounds of iron that do not contain substituents at or C, the T values are as follows t(H ) = 9.5-9.8 and t(H ) = 8.0-8.4. Substituents somewhat decrease the t values. Based on the NMR spectra it is possible to determine if the diene is coordinated in the transoidal or cisoidal form. In complexes (8.5), the /(2,3) 

---