Molybdenum complexes cyclobutadiene

Some reports of failures to obtain cyclobutadiene complexes have also appeared (10,40). Chatt et al. (21) also noted that biphenylene (II) was very reluctant to form a metal complex. The only ones which could be made were molybdenum tricarbonyl complexes where the Mo(CO)3 group was bonded to the benzene, rather than to the four-membered ring. 

The only bis(cyclobutadiene)-metal complex so far described is the molybdenum one, (XXIII). The reason for the lack of other analogs of... 

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