The most polymorphic metal thiocyanates are the monovalent group 10 metals, Cu(NCS) and Ag(NCS), which have three and two distinct polymorphs (Fig. ). Cu(NCS) has tetrahedral Cu(I), coordinated by the S and one N atom, and equally each S atom coordinates to three Cu and each N to one Cu, i.e. NCS -is also tetrahedrally coordinated. Like other binary tetrahedral compounds this leads to extensive polymorphism. There are three reported crystal structures: orthorhombic Pbca α-Cu(NCS) , P6 3 mc 2H-β -Cu(NCS) and R3m 3R-β -Cu(NCS). All of these compounds have in common honeycomb-like CuS layers. 2H-β and 3R-β have thiocyanates all point in the same direction, yielding polar structures, and in the α phase the thiocyanates alternate up and down in a stripe order pattern, producing a non-polar structure. The two distinct β polymorphs (which have been confusingly allocated the same Greek letter label) differ only in the stacking of the CuS layers relative to each other, i.e. they are polytypes. 2H-β has AB stacking, analogous to wurzite, and 3R-β has ABC stacking, analogous to sphaelerite. The various polymorphs are all slightly soluble in water, and can be prepared through various routes, including slow evaporation of solutions of Cu(NCS) in aqueous Na(NCS) solution and spontaneous reduction of aqueous Cu(NCS) 2 (NH 3 ) 2 solutions. There are no reports of phase transitions on heating or cooling in Cu(NCS), with powder diffraction measurements from 80 to 400 K on 2H-β showing no phase-change and modest thermal expansion, (α a = 10.5 MK -1 , α c = -1.2 MK -1 ). Indeed, the thermal expansion is considerably smaller than predicted through DFT using the quasiharmonic approximation. High pressure Raman spectroscopy and powder X-ray diffraction measurements suggest that 2H-β phase transforms slowly into α above 4 GPa, before irreversibly amorphising through polymerization. Cu(NCS) is of particular interested as the extended CuS layers allow for significant electronic delocalization and hence ptype transparent conduction. Cu(NCS) is solution processable and has been explored as a hole transport layer in solar cells. Ag(NCS) has two reported polymorphs. β -Ag(NCS) has a structured related to that of α-Cu(NCS) and comprises AgS 3 N 1 tetrahedra connected by thiocyanates into a tetrahedral network, with a honeycomb type AgS layer with alternating up-and-down thiocyanates. α-Ag(NCS) contains AgS 3 N tetrahedra, but is quite distinct from the other structures. The tetrahedra edgeshare, and the bond lengths vary significantly. It can alternatively be thought of as consisting of 1D chains running along approximately the [101] direction, analogous to the structure of Ag(CN), connected by longer Ag-S into a 3D network. α-Ag(NCS) has a shortest Ag-Ag distance of 3.28 Å and two further Ag atoms at 3.30 and 3.41 Å, strongly suggestive of significant argentophilic interactions, common in the silver cyanides. The argentophilic interactions form a 3D network with the srs or (10,a) topology and it is likely that these interactions are responsible for the unique structure of α-Ag(NCS). Ag(NCS) is very insoluble and can be prepared by precipitation from aqueous solution from soluble Ag + and NCS -salts. β -Ag(NCS) is favoured by rapid precipitation, acidification and the presence of gelatine. Variable temperature powder neutron diffraction study of the α phase on cooling shows small positive thermal expansion along the a direction, and larger expansion along b and c directions (4.0 MK -1 , 30 MK -1 and 54 MK -1 respectively), but no phase transitions were observed down to 25 K.