The redox chemistry of [(Cp ''' Co)(2)(mu,eta(2):eta(2)-E-2)(2)] (E=P (1), As (2); Cp '''=1,2,4-tri(tert-butyl)cyclopentadienyl) was investigated. Both compounds can be oxidized and reduced twice. That way, the monocations [(Cp ''' Co)(2)(mu,eta(4):eta(4)-E-4)][X] (E=P, X=BF4 (3 a), [FAl] (3 b); E=As, X=BF4 (4 a), [FAl] (4 b)), the dications [(Cp ''' Co)(2)(mu,eta(4):eta(4)-E-4)][TEF](2) (E=P (5), As (6)), and the monoanions [K(18-c-6)(dme)(2)][(Cp ''' Co)(2)(mu,eta(4):eta(4)-E-4)] (E=P (7), As (8)) were isolated. Further reduction of 7 leads to the dianionic complex [K(18-c-6)(dme)(2)][K(18-c-6)][(Cp ''' Co)(2)(mu,eta(3):eta(3)-P-4)] (9), in which the cyclo-P-4 ligand has rearranged to a chain-like P-4 ligand. Further reduction of 8 can be achieved with an excess of potassium under the formation of [K(dme)(4)][(Cp ''' Co)(2)(mu,eta(3):eta(3)-As-3)] (10) and the elimination of an As-1 unit. Compound 10 represents the first example of an allylic As-3 ligand incorporated into a triple-decker complex.