Periodical trends in [Co6E(CO)16]-clusters : Structural, synthetic and energy changes produced by substitution of P with As

The anionic cluster [Co6As(CO)16]- was synthesized through the reaction of Na[Co(CO)4] and arsenic acid in THF at room temperature. Crystallization from MeOH/2-propanol yielded two polymorphs that feature two slightly different isomers of the anion with the same PPh4+ cation. One of the isomers is very similar to the known [Co6P(CO)16]-, formed by four edge fused triangles, partially wrapping the main group atom. The other isomer features a deformed cage, which differs mainly for a non-bonding Co-Co distance. The reasons for this unprecedented stereochemistry, and the factors which may trigger this isomer, have been investigated by DFT calculations.