Quantum chemical calculations as well as crystallographic analyses show that the Ni rings in the tiara Ni-n thiolates, [NiS2](n) and [Ni(SR)(2)]n (n = 3-6), have highly symmetric polygonal structures. We find that such structural features primarily arise from the effective delocalization of the d-orbital electrons across the Ni rings leading to bond length equalization and thereby aromaticity. We introduce the d-orbital aromaticity for the first time to explain the experimentally observed polygonal structures of these cyclic metal rings bridged by thiol linkages.