For tetrahedral metals bound by thiocyanate, the 1:1 hard:soft stoichiometry typically produces a diamondoid net, comprising alternating MN 4 and MS 4 tetrahedra (Fig. ). This structure type was first determined by Jeffery for CoHg(SCN) 4 , historically widely used in microanalysis 170 and as a calibrant for measures of magnetic susceptibility, though there is both a significant easyplane zero field splitting, with D = +10.6(5) cm -1 , and weak superexchange zJ = -0.2(1) cm -1 . As this family crystallizes in the polar I4 space group, these compounds are also of interest for their non-linear optical properties, particularly ZnHg(SCN) 4 , 171 MnHg(SCN) 4 and FeHg(SCN) 4 . The structure is now known for M[Hg(SCN) 4 ] for M = Zn, 171 Co, Fe, 172 Mn, 172 Cd; and Zn[Cd(SCN) 4 ]. Cu[Hg(SCN) 4 ] adopts a related but distinct structure containing square planar Cu(NCS) 4 ions, which has two subtly different monoclinic and orthorhombic polymorphs. Ni[Hg(SCN) 4 ] is not known, with hydrates and methanolates forming instead (likely due to the difficulty of preparing tetrahedral Ni(II)). The crystal structure of a related composition, NiHg 2 (SCN) 6 , has been reported but the unphysical NiN 6 coordination sphere, with N-N distances as short as 1.64 Å, and N-Ni-N angles of 50 • suggests the structure is incorrect. The true structure may be related to the complex structure of MnHg 2 (SCN) 6 (Section 3.4). 180