A synthetic pathway for the synthesis of novel anionic sandwich complexes with a cyclo-E3 (E=P, As) ligand as an end deck was developed giving [Cp′′′Co(η3-E3)]− (Cp′′′=1,2,4-tri-tert-butylcyclopentadienyl, E=P ([5]), As ([6])) in good yields suitable for further reactivity studies. In the reaction with the chlorophosphanes R2PCl (R=Ph, Cy, tBu), neutral complexes with a disubstituted cyclo-E3P (E=P, As) ligand in [Cp′′′Co(η3-E3PR2)] (E=P (7 a–c), As (9 a–c)) were obtained. These compounds can

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... partially or completely converted into complexes with a cyclo-E3 (E=P, As) ligand with an exocyclic {PR2} unit in [Cp′′′Co(η2:η1-E3PR2)] (E=P (8 a–c), As (10 a–c)). Additionally, the insertion of the chlorosilylene [LSiCl] (L=(tBuN)2CPh) into the cyclo-E3 ligand of [5] and [6] was achieved and the novel heteroatomic complexes [Cp′′′Co(η3-E3SiL)] (E=P (11), As (12)) could be isolated. The reaction pathway was elucidated by DFT calculations.