Unusual metal centres/coordination spheres in spin crossover compounds

•The SCO chemistry of complexes containing Cr2+, Mn2+, Mn3+ and Co2+ is reviewed.•A structural analysis of the M-L bond lengths and angles upon SCO is presented.•Co2+ complexes can undergo SCO in octahedral and five-coordinate spheres.•A “magic” coordination sphere for Mn3+ complexes has been stablished, N4O2. The most widely studied spin crossover (SCO) systems are those involving hexacoordinate iron(II) complexes immerse in the “magic” N6 coordination sphere. In this review the properties of SCO-active metal complexes containing unusual metal centres and/or coordination spheres are analysed, except for Fe3+ complexes that have been recently reviewed. The SCO-properties of the less studied SCO metal centres Cr2+, Mn2+, Mn3+ and Co2+ are presented, showing great advances in the development of bistable materials containing first row transition metals other than Fe. Finally, an analysis of the changes in the structural parameters, bond lengths and angles, during the spin conversion is presented for those complexes structurally characterised by X-ray crystallography in both high- and low-spin states.