The structures and bonding of alkali metal thiocyanates are much more ionic than other metal thiocyanates (thallium has been included in this section due to the similarity of its chemistry) (Fig. ). As metal pseudohalides, they adopt structures similar to the NaCl (Na, Li) and CsCl structures (K, Rb, Tl, Cs) . The ionic bonding means that order-disorder transitions are a common feature of these compounds, like other metal pseudohalides, with phase transitions to plastic phases reported for K(NCS), Rb(NCS), Tl(NCS) and Cs(NCS). Sodium and lithium thiocyanate both adopt rocksalt-like structures with space group Pmcn, in which both the N and S termini are bonded to three metal ions, and each metal ion is coordinated (c) Cs(NCS) has a different CsCl derived structure which transforms into a cubic phase shortly before melting. by three N and three S atoms. Phase transitions are reported to be absent for Na(NCS) on heating or cooling, or on pressure up to 4 GPa. Lithium thiocyanate is an Li ion conductor due to the presence of Li vacancies in typically synthesized samples. K(NCS), Rb(NCS) and Tl(NCS) all adopt the same ordered orthorhombic Pbcn structure at room temperature, with eight coordinate NCS, four through the N terminus and four through the S, and eight coordinate metal, both in a distorted cubic coordination. On heating, T c = 415 K for K, 434 K for Rb and 383 K for Tl, a high temperature I4/mcm tetragonal structure forms, closely related to the low temperature form, but with head-tail NCS disorder. The weakly first order transition has been extensively studied for K(NCS) and is well approximated by a pseudospin model. On compression, a series of additional phases have been reported to exist from thermal analysis and Raman spectroscopy measurements. This suggests that in addition to the room-temperature ambient ordered II phase and high temperature I phase, there are five additional high pressure phases, though there are as yet no diffraction studies confirming their structures. Cs(NCS) has, like the other larger metal thiocyanates, a structure related to the CsCl structure, with eight coordinate Cs + and NCS -. However, both the low temperature ordered phase and high temperature phase are different from the K(NCS) structure type, with a more distorted low temperature Pnma phase and a high temperature maximally symmetric Pm3m cubic phase, with complete orientational disorder of the NCS -anion. The orderdisorder transition is first order and the high temperature cubic phase exists only in a narrow temperature window, between 470 K and its melting point of 479 K. The symmetry relationships between the Cs(NCS) and K(NCS) structure types, and their phase transitions have been determined by Shlyaykher et al.. Na(NCS) and K(NCS) are both readily commercially available. Rb(NCS) and Cs(NCS) can be prepared either by reaction of NH 4 (NCS) with M 2 (CO 3 ) in water, or by salt metathesis between M 2 (SO 4 ) and Ba(NCS) 2 and filtering off the insoluble BaSO 4 . Li(NCS) • H 2 O is also commercially available, and the anhydrate can be prepared through careful dehydration. Although this review focusses on the solid metal thiocyanates, it is worth noting that the low melting points of ionic metal thiocyanates means that they have significant liquid state chemistry as molten salts/ionic liquids. Recent reports also suggest that solid solutions of Cu(I) and Zn(II) thiocyanates may also melt, though inhomogeneously.