Cyclobutadiene and Its Metal Complexes

The practical and theoretical problems associated with the cyclobutadiene molecule have concerned organic chemists for over one hundred years and, as is well known, this compound has played a particularly interesting role in the development of organic chemistry. The importance of the system to organic chemists stems mainly from the fact that the molecule is a cyclic conjugated system for which, seemingly, two equivalent structures (Ia and Tb) could be written, just as two such structures (ha and JIb) can be written for benzene. Kekulé and many of his contemporaries associated the peculiar