Chromacyclobutenes 2, which have been postulated as intermediates on several occasions, are unrealistic structures with incomplete electron shells and high energy. According to model calculations vinylcarbene complexes 3/4 are much more stable isomers and can be formed almost without activation energy from alkyne-carbene complexes 1. The different modes of coordination of the structure continuum 1–6 can be interpreted and predicted as a function of the metal complex MLn.
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Hofmann et al. (1989) studied this question.
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