PtZnH5-, A σ-Aromatic Cluster

We report a joint photoelectron spectroscopic and theoretical study of the PtZnH5- cluster anion. This cluster exhibited an unprecedented planar pentagonal coordination for Pt and an unusual stability and high intensity in the mass spectrum. Both are due to the sigma-aromaticity found in the H-5-cycle supported by the 5d orbitals on the Pt atom. sigma-Aromaticity in all-H systems has been predicted in the past but never found in experimentally observed species. Besides fundamental importance, mixed transition-metal hydrides can be found as intermediates in catalytic processes, and thus, the unexpected stability facilitated by sigma-aromaticity can be appreciated also in practical applications.