The crystal structure of Hg(C₆F₄-o-NO₂)₂1 is planar with intramolecular Hg–O coordination whereas those of the polymorphs of Hg(C₆F₄-m-NO₂)₂2a and 2b have the two aryl rings rotated relative to each other (87° and 59°). At a supramolecular level, both complexes exhibit weak Hg⋯O interactions. Theoretical calculations (B3LYP and MP2) for gas-phase Hg(C₆F₄-o-NO₂)₂ and Hg(C₆F₄-m-NO₂)₂ molecules predict non-planar conformations in both complexes corresponding to one polymorph of the latter (2a). However, when the supramolecular Hg⋯O interactions are included, using a dimeric model, the calculations more accurately predict the observed structures of 1 and 2b.