Reaction of achiral 2,5-diphenyl-3,4-di(3-pyridyl)cyclopenta-2,4-dien-1-one (2) with ZnCl2 and HgBr2, respectively, afforded the helically chiral coordination polymers [(2)ZnCl2]infinity and [(2)HgBr2]infinity, which show spontaneous chiral resolution, forming colonies of homochiral crystals.

The aromatic compounds fluorenone (1) and 4,5-diazafluoren-9-one (2) behave like cyclopentadienones in their reaction with [Cp*Co(C 2 H 4 ) 2 ] (3a), affording the sandwich-type complexes [Cp*Co(η 4 -1)] ( 4) and [Cp*Co(η 4 -2)] ( 5), respectively, whose structures have been determined by X-ray diffraction. Both complexes show double-bond fixation in their η 2 -coordinated sixmembered rings and belong to a new structure type of mononuclear organometallics. The cobalt-coordinated ligands 1 and 2 present in 4 and 5, respectively, may be replaced by two molecules of carbon monoxide. In accord with their cyclopentadienone-like nature, they are nucleophilic and undergo O-acylation with acetyl chloride, affording cobaltocenium species.

2,5-Diphenyl-3,4-bis(2-pyridyl)cyclopenta-2,4-dien-1-one (1), a close relative of tetraphenylcyclopentadienone, is a new ligand platform for use in redox switches and sensors. Compound 1 acts as a molecular electrochemical sensor towards a range of divalent metal ions and exhibits favourable two-wave behaviour. It forms chelates of the type [(1)MX2], whose stability is enhanced by five orders of magnitude upon one-electron reduction. The bite angle of 1 is close to 90 degrees in these complexes. The attachment of the 14-valence-electron Cp*Co fragment to the cyclopentadienone pi system reduces the bite angle and thus modulates the binding characteristics of 1.

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