Dicyclopentadienylnickel: A problematical reagent in cluster synthesis

The room temperature reaction of (C5H5)2Ni with Co3(CO)9CR, where R is CO2CH(CH3)2, gives almost quantitative replacement of a Co(CO)3 group by a (C5H5)Ni fragment. However, the same reagents in refluxing THF do not lead to further replacement of Co by Ni but instead yield the products (C5H5)3-Co3(CO)(H)CR and (C5H5)3Co2Ni(CO)CR in which carbonyl groups have been replaced by cyclopentadienyl ligands. The isolobal nature of the fragments in these complexes is noted.