A SQUARE AS-4 AND A PRISMATIC AS-6 STRUCTURE AS COMPLEX LIGANDS

[As-7(SiMe3)(3)] reacts with [(CpCoCl)(2)] to give black crystals of the ionic compound [(CpCo)(3)As-6][CoCl4] (1). The cation [(Cp*Co)(3) As-6](2+) consists of an As-6 prism, with squares capped by [CpCo] fr agments, When [((CpCoCl)-Co-tBu)(2)] is used instead of [(CpCoCl)(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[Co3Cl 8(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 c an be described as a triple-decker sandwich complex with two [(CpCo)-C o-tBu](+) fragments bridged by an As-4 ligand. Density functional calc ulations reproduce the experimental data of 1 and 3 and allow an inter pretation of molecular electronic structure and bonding in these and r elated compounds with As replaced by P and Co by Fe.