Comparison of the crystal structures of the low- and high-temperature forms of bis-[4-(di-methyl-amino)-pyridine]-dithio-cyanato-cobalt(II).

Single crystals of the high-temperature form I of [Co(NCS)(2)(DMAP)(2)] (DMAP = 4-di-methyl-amino-pyridine, C(7)H(10)N(2)) were obtained accidentally by the reaction of Co(NCS)(2) with DMAP at slightly elevated temperatures under kinetic control. This modification crystallizes in the monoclinic space group P2(1)/m and is isotypic with the corresponding Zn compound. The asymmetric unit consists of one crystallographically independent Co cation and two crystallographically independent thio-cyanate anions that are located on a crystallographic mirror plane and one DMAP ligand (general position). In its crystal structure the discrete complexes are linked by C-H⋯S hydrogen bonds into a three-dimensional network. For comparison, the crystal structure of the known low-temperature form II, which is already thermodynamically stable at room temperature, was redetermined at the same temperature. In this polymorph the complexes are connected by C-H⋯S and C-H⋯N hydrogen bonds into a three-dimensional network. At 100†K the density of the high-temperature form I (ρ = 1.462†g†cm(-3)) is higher than that of the low-temperature form II (ρ = 1.457†g†cm(-3)), which is in contrast to the values determined by XRPD at room temperature. Therefore, these two forms represent an exception to the Kitaigorodskii density rule, for which extensive inter-molecular hydrogen bonding in form II might be responsible.