The Cobalt cyclo-P4 Sandwich Complex and Its Role in the Formation of Polyphosphorus Compounds.

A synthetic approach to the sandwich complex [Cp'''Co(η4 -P4 )] (2) containing a cyclo-P4 ligand as an end-deck was developed. Complex 2 is the missing homologue in the series of first-row cyclo-Pn sandwich complexes, and shows a unique tendency to dimerize in solution to form two isomeric P8 complexes [(Cp'''Co)2 (μ,η4 :η2 :η1 -P8 )] (3 and 4). Reactivity studies indicate that 2 and 3 react with further [Cp'''Co] fragments to give [(Cp'''Co)2 (μ,η2 :η2 -P2 )2 ] (5) and [(Cp'''Co)3 P8 ] (6), respectively. Furthermore, complexes 2, 3, and 4 thermally decompose forming 5, 6, and the P12 complex [(Cp'''Co)3 P12 ] (7). DFT calculations on the P4 activation process suggest a η3 -P4 Co complex as the key intermediate in the synthesis of 2 as well as in the formation of larger polyphosphorus complexes via a unique oligomerization pathway.