In a number of cases, alkenes that are too unstable for isolation have been
isolated in the form of metal complexes. As example is norbornadienone,
which was isolated in the form of its iron–tricarbonyl complex (13),92 where
the norbornadiene unit is an Z4 ligand, and each of the carbonyl units are Z2
ligands. The free dienone spontaneously decomposes to carbon monoxide
and benzene (see reaction 17-28).