Construction of double- and triple-decker sandwich compounds from half-sandwich compounds: a theoretical assessment

The viability and properties of double- (Cp2M) and triple-decker (Cp3M2) sandwiches formed from half-sandwiches (CpM, Cp = C5H5; M = Li, Na, K, Be, Mg, Ca, Fe, Co, Ni, Cu, and Zn) are discussed based on the geometry, energy, HOMO-LUMO gap, and topological properties. The calculated results show that the alkali metals and transitional metals (Fe, Co, Ni) with more unpaired electron are more inclined to form high-symmetry sandwich complexes than the alkaline earth metals. The Cp2M and Cp3M2 symmetries for M = Cu and Zn are low. In Cp2M and Cp3M2, the electrostatic and π orbital interactions are dominant. For Cp3M2, the contributions of orbital interaction to the total M-C interaction and of σ-type interaction to the orbital interaction are larger than those in Cp2M. The nature of the M-C bond is well correlated to its bond length. The shorter the M-C bond, the more covalent it is.