A square As-4 and a prismatic As-6 structure as complex ligands

[As-7(SiMe3)(3)] reacts with [(Cp*CoCl)(2)] to give black crystals of the ionic compound [(Cp*Co)(3)As-6][CoCl4] (1). The cation [(Cp*Co)(3)As-6](2+) consists of an As-6 prism, with squares capped by [Cp*Co] fragments, When [((CpCoCl)-Co-tBu)(2)] is used instead of [(Cp*CoCl)(2)] in the reaction, dark red crystals of [((CpCo)-Co-tBu)(3)As-6][CoCl3(thf)](2) (2) and green-black crystals of [((CpCo)-Co-tBu)(2)As-4[Co3Cl8(thf)(2)] (3) can be obtained. The cation in 2 shows a structure very similar to that of 1. The [((CpCo)-Co-tBu)(2)As-4](2+) cations in 3 can be described as a triple-decker sandwich complex with two [(CpCo)-Co-tBu](+) fragments bridged by an As-4 ligand. Density functional calculations reproduce the experimental data of 1 and 3 and allow an interpretation of molecular electronic structure and bonding in these and related compounds with As replaced by P and Co by Fe.