The reaction of cobalt(II) chloride hexahydrate and pyrazine, in a reduced amount of water under hydrothermal conditions affords [CoCl2pyz]n, which crystallises in an orthorhombic unit cell (a = 12.0014, b = 7.1040, c = 3.5380 Å), space group Cmmm. FTIR confirms the coordination of pyrazine to Co(II) in an octahedral geometry. This compound is thermally stable in air up to 320 °C, and may also be obtained by the single-step decomposition of [CoCl2(pyz)2]n at 190 °C, with an activation energy of 121 kJ mol-1. [CoCl2pyz]n contains chains of Co(II) ions bridged by two chloride ions making an angle of 92.21º, leading to orthogonal (d-p) orbitals and to one-dimensional ferromagnetism. In contrast, in [CoCl2(pyz)2]n only pyrazine bridges the Co(II) ions, setting them apart, resulting in a purely paramagnetic system. Because the simple contact with pyrazine vapour or aqueous pyrazine reintroduces this molecule into [CoCl2pyz]n, while heating [CoCl2(pyz)2]n removes pyrazine affording [CoCl2pyz]n, the magnetic behaviour may be reversibly switched between the two states.