Six metal complexes of Schiff bases involving Vitamin B₆ and the decarboxylated amino acid histamine have been synthesised and characterized. Crystal structures have been determined for [CuL¹(H₂O)Br]-NO₃1(L¹= pyridoxylidenehistamine) and [Cu₂L²₂(NO₃)₂]·6H₂O 2(L²= 5′-phosphopyridoxylidenehistaminate). The crystal structure of complex 1[space group P, a= 8.161(2), b= 10.368(2), c= 11.110(2)Å, α= 105.18(1), β= 102.12(1), γ= 72.10(1)° and Z= 2; R= 0.072, R′= 0.083] consists of square-pyramidally co-ordinated copper with the tridentate Schiff base in the zwitterionic form, whereas in 2[space group P, a= 8.727(1), b= 10.308(1), c= 12.845(2)Å, α= 110.00(1), β= 78.94(1), γ= 114.35(1)° and Z= 1; R= 0.035, R′= 0.034] the copper has the same co-ordination geometry but the tetradentate Schiff-base ligand exists as a monoanion. The conformational parameters deduced from such structures are important for understanding the stereochemical aspects of Vitamin B₆-catalysed model reactions involving histidine.