A spin crossover ferrous complex with ordered magnetic ferric anions.

The first tetrahaloferrate spin crossover compound, [Fe(Metz)(6)](FeBr4)(2) (Metz = 1-methyltetrazole), is reported. The FeBr4- ions form ferromagnetically coupled 1D stacks and exhibit an antiferromagnetic order at 2.2 K, which coexists with the gradual spin crossover centred at 165 K.