Butadiene and Cyclobutadiene Complexes 4 7r-Systems

When both double bonds of butadiene are coordinated, there is generally a shift to lower frequencies (Table 4-4). 

A wide variety of other conjugated diolefins have been shown to form (C4)M-type bonding systems. These include cyclopentadiene, cyclopenta-dienone, cyclohexadiene, cycloheptatrienone, and cyclooctatetraene. The bands attributed to the free olefin, appearing at 1575-1600 cm Mn the free olefins, display a typical bathochromic shift (to 1500-1460 cm ) in their complexes. If one double bond of the diolefin is complexed while the second remains free, often two well-separated absorptions are observed, one being in the region of an uncomplex olefinic linkage and the other at a lower frequency (Table 4-5). 

The tetramethyl-substituted derivatives have proven more susceptible to IR studies. The original nickel chloride derivative has been shown by an x-ray study to be dimeric with two chlorine-bridging atoms and planar four-membered rings centrosymmetrically bonded to the nickel atoms. After assigning the well-known methyl vibrations, those arising from the (C4)M system and Ni-Cl bonds have been established (Table 4-6). 

These apparently inconsistent results have been interpreted by postulating a rapid rearrangement from one (C4)Fe bonding system to the opposite one in solutions of this complex. 

The C5H5 group can be attached to a metal in four different bonding modes ionically bonded, centrally cr-bonded, centrally 7c-bonded, and cr-bonded with a diene structure. 

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