Mono and bis iron(11) complexes of 2,4-bis(2-pyridyl)thiazole have been described. Conductivity and visible and infrared spectral data suggest that the mono complexes are five-coordinate. The magnetism of the bis complexes, in contrast to that of the mono complexes, shows unusual temperature dependence. This has been ascribed to the presence, in the bis complexes, of a spin-state equilibrium. Bis[2,4-bis(2-pyridyl)thiazole]cobalt(11) perchlorate has been shown to be a simple paramagnetic compound.