Neutral radicals, mixed-ligand

Unlike stable V(CO)6, neutral M(CO)6 (M = Ta, Nb) have not been characterized (except in matrix-isolation experiments, see Matrix Isolation). Neutral mixed-ligand complexes were unknown until 1996. The first Ta(0) carbonyl complex Ta(CO)4(dppe) was synthesized by the reaction of hydride TaH(CO)4(dppe) with the (p-Bu C6H4)3C- radical. This complex exists as a CO-bridged dimer in solid and, in solution, as an equilibrium mixture of the monomer and dimer. ... 

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