Synergy between Experimental and Computational Chemistry Reveals the Mechanism of Decomposition of Nickel–Ketene Complexes

A series of (dppf)­Ni­(ketene)
complexes were synthesized and fully
characterized. In the solid state, the complexes possess η2-(C,O) coordination of the ketene in an overall planar configuration.
They display similar structure in solution, except in some cases,
the η2-(C,C) coordination mode is also detected.
A combination of kinetic analysis and DFT calculations reveals the
complexes undergo thermal decomposition by isomerization from η2-(C,O) to η2-(C,C) followed by scission of
the CC bond, which is usually rate limiting and results in
an intermediate carbonyl carbene complex. Subsequent rearrangement
of the carbene ligand is rate limiting for electron poor and sterically
large ketenes, and results in a carbonyl alkene complex. The alkene
readily dissociates, affording alkenes and (dppf)­Ni­(CO)2. Computational modeling of the decarbonylation pathway with partial
phosphine dissociation reveals the barrier is reduced significantly,
explaining the instability of ketene complexes with monodentate phosphines.