A dimercury(I) 18-crown-6 complex is isolated, and its possible role in the hydrothermal preparation of the mercuric nitrite complex is discussed. The reported structures are of [Hg-2(18-crown-6)(2)(H2O)(2)](ClO4)(2) (1), monoclinic, C2/c, a = 21.0345(9), b = 12.1565(5), c = 16.8010(7) angstrom, beta = 113.2000(10)degrees, V = 3948.7(3) angstrom(3), Z = 16, R = 0.0230; [Hg(18-crown-6)](NO2)(2) (2), monoclinic, P2(1)/c, a = 8.027(5), b = 14.437(9), c = 7.827(5) angstrom, beta = 95.165(11)degrees, V = 905 6(10) angstrom(3), Z = 2, R = 0.0175. The complex cation in compound 1 consists of a mercurous dimer exhibiting a Hg-Hg bond length of 2.524(2) angstrom. Non-bonding interactions between adjacent crown ether macrocycles across the Hg-Hg bond result in large variations in mercury to oxygen distances within equatorial coordination sites. At low pH compound 1 is proposed to be preferentially formed under hydrothermal conditions affording compound 2 upon disproportionation. Nitrite ions ligate via a unidentate nitrito (cis to metal) coordination mode as interpreted using vibrational (infrared) spectroscopy. The conformation adopted by 18-crown-6 in compounds 1 and 2 is closely related to a D-3d conformation as evidenced by X-ray crystallography. Band splitting readily observed in vibrational spectra of the metal free crown ether, attributed to vibrational modes of oxyethylene fragments, is absent in spectra of 1 and 2 confirming a regular D-3d macrocyclic orientation. Short Hg-O bonds observed for axially coordinated water molecules in 1 and coordinated nitrite ligands in 2, illustrate the prevalence of relativistic effects commonly observed in mercury complexes.