Abstract The structures of complexes of the type M II (Hal) 2. 2 Pyridine have been investigated by a variety of physical techniques. Particular attention has been paid to the cobalt complexes which can exist in one or both of two isomeric forms. The factors which decide whether the tetrahedral monomer or octahedral polymer form will be the more stable are discussed. By using various metals five different types of complexes of the above formula have been identified — (a) tetrahedral monomer; (b) octahedral polymer; (c) square planar monomer; (d) tetragonal polymer; (e) (presumably) irregular tetrahedral monomer. The influence of the electronic configuration of the metal and the effect of the ligand on the structure adopted are discussed.