The most common structure type for binary metal thiocyanates is the layered Ni(NCS) 2 structure, which is found for Ni, Co, Fe, Mn and with some variation, Hg and Cu 2+ (Fig. ). This structure is analogous to the CdI 2 structure type common for metal halides, comprising layers of edge-sharing MN 2 S 4 oc- tahedra. The neutral layers are held together by van der Waals forces. The aristotype structure crystallizes in space group C2/m, lowered from the trigonal symmetry of the analogous halides due to the rod-like shape of thiocyanate. The M-SCN angle is around 100º, and the M-NCS angle is roughly 160º. Hg(II) and Cu(II) have structural deviations. For Hg(II), the tendency to lower coordination numbers and its soft nature means that there are two shorter Hg-S bonds, and four long Hg-N bonds, meaning there is instead HgS 2 N 4 pseudo-octahedral coordination, or nearly linear HgS 2 coordination. Cu(II) thiocyanate has a pronounced Jahn-Teller distortion which lengthens one set of Cu-S bonds, lowering the symmetry to P1 and producing a near one-dimensional structure. These compounds can in general be prepared by salt metathesis, either in water from metal sulfate and Ba(NCS) 2 , or in an organic solvent from e.g. M(BF 4 ) 2 and KNCS, with the insoluble by-product filtered off and then the removal of the solvent from the filtrate containing M(NCS) 2 . Typically this also requires desolvation of a solid solvate. Hg(NCS) 2 and Cu(NCS) 2 are both insoluble in water and can be prepared by direct reaction of a soluble metal salt and thiocyanate source. The synthesis of Cu(NCS) 2 can be challenging, due to the instability of this compound to Cu(NCS), and it can only be prepared at high concentrations. It will disproportionate on prolonged exposure to water forming Cu(NCS) and a mixture of by-products. The difficulty of preparing pure phase Cu(NCS) 2 meant its structure was determined 180 years after the first report of its existence. Fe(NCS) 2 is the least chemically stable of the reported Ni(NCS) 2 structured frameworks, oxidising in air, although other compounds are sensitive to moisture: Mn(NCS) 2 is deliquescent and Co(NCS) 2 can be hydrated under humid conditions. Thermogravimetric analysis suggests that these compounds are thermally stable in inert atmosphere up to approximately 300 • C, though Hg(NCS) 2 has long been known for its notable for its spectacular thermal decomposition in air to carbonitrides and mercury sulfides and oxides, the so-called 'Pharaoh's Serpent' reaction. These frameworks are also the only characterized binary thiocyanates containing paramagnetic ions (there are no magnetic measurements of Eu(NCS) 2 ). The first property measurements of Mn(NCS) 2 , Ni(NCS) 2 and Co(NCS) 2 included magnetic susceptibility measurements over limited temperature range, which indicated significant antiferromagnetic interactions. These initial measurements were built upon, and long-range antiferromagnetic order has now been found in Ni(NCS) 2 , Co(NCS) 2 , Cu(NCS) 2 , 30 Mn(NCS) 2 and Fe(NCS) 2 . Subsequent neutron diffraction revealed unusual layered antiferromagnetism in Ni(NCS) 2 , , and density functional theory calculations suggest that these materials, like the analogous metal halides, may be delaminated to produced 2D magnets. Despite the propensity for transition metals to adopt a variety of oxidation states, only Cu has fully characterized compounds in an oxidation state other than +2 (Section 3.2). Hg(NCS) has been reported to form, as has Fe(NCS) 3 , though nothing beyond their apparent composition is known. This family has series of unusual pharmacological functions, acting as both poacher and gamekeeper: Co(NCS) 2 can be used as a test for the presence of cocaine, and Cu(NCS) 2 is a component in salts designed to obscure the presence of the drug. 53,54