The bonding of 4-eIectron ligands to transition metals

The classification of the metal and cyclobutadiene orbitals according to their symmetry properties as members of the group C4v is given in Table 8. 

Symmetry Ligand orbital Metal orbital Bond type 

Unfortunately, even the local symmetry of the butadiene-metal system is low (Cj) and, as shown in Table 9 which gives the transformation 

The metal orbitals which are most likely to be important in the diene-metal bond 

There is little evidence to distinguish between A, B or C. X-ray studies of a number of diene iron tricarbonyl complexes have been made.f A comparison of the observed bond distances between the terminal carbons of diene systems and the adjacent carbons of a substituent group (1-44-1 49 A) with the calculated distances for an s/ hydridized terminal carbon (1-48 A) and an sp hybridized carbon atom (1-52-1-54 A) slightly favours structures A and C [16]. The C -H coupling constants of butadiene iron tricarbonyl are more consistent with sp hybridized carbons than with hybridization [16a]. The data also suggests that there is a small rotation of the CH2 hydrogens out of the C4-plane. 

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