A synthetic pathway for the synthesis of novel anionic sandwich complexes with a cyclo-E-3 (E=P, As) ligand as an end deck was developed giving [Cp ''' Co(eta(3)-E-3)](-) (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, Bu-t), neutral complexes with a disubstituted cyclo-E3P (E=P, As) ligand in [Cp ''' Co(eta(3)-E3PR2)] (E=P (7 a-c), As (9 a-c)) were obtained. These compounds can be partially or completely converted into complexes with a cyclo-E-3 (E=P, As) ligand with an exocyclic {PR2} unit in [Cp ''' Co(eta(2):eta(1)-E3PR2)] (E=P (8 a-c), As (10 a-c)). Additionally, the insertion of the chlorosilylene [LSiCl] (L=((BuN)-Bu-t)(2)CPh) into the cyclo-E-3 ligand of [5] and [6] was achieved and the novel heteroatomic complexes [Cp ''' Co(eta(3)-E3SiL)] (E=P (11), As (12)) could be isolated. The reaction pathway was elucidated by DFT calculations.