5. Stabilization of unstable species by complexation

It was seen previously that cyclobutadiene can be formed by the action of an alkyne on the iron carbonyl and remains bound to the iron. It was also seen that PCl ₃ can be transformed into a P ₂ molecule that remains complexed and is impossible to obtain in the free state.

Examples of this kind open the door to unknown fragments which can then serve as a starting point for further reactions.

Cyclobutadiene can also be obtained from 3,4-dichlorocyclobut-1-ene in the presence of carbonyl iron. The iron acts both as a stabilizer for cyclobutadiene and as a trap for chlorine atoms (figure 56 ):