The structural nature of transition metal ion complexes of heterocyclic thiosemicarbazones and S alkyl 1/aryl 1 dithiocarbazates has been correlated to their wide spectrum therapeutic uses. In continuation of our work on structural characterisation of first row transition metal ion complexes of biologically relevant pyrazole- and pyrimidine-derived ligands, the communication reports the synthesis, spectroscopy and X-ray crystal structure of a copper(II) chloride complex of the title ligand (HMPzSB).HMPzSB (C 1 3 H 1 4 N 4 S 2 ), synthesised and spectroscopically characterised (mass, IR, NMR), reacts with hot ethanolic solution of copper(II) chloride to yield a green microcrystalline complex which analyses as [Cu(C 1 3 H 1 2 N 4 S 2 .OC 2 H 5 )Cl]. The non-electrolytic Cu(II) complex with RT magnetic moment of 1.81 B.M. and a characteristic d-d band at ca. 640 nm in the electronic spectrum is proposed to have square planar structure. The RT ESR parameters (g > g ) show an axial d x 2 - y 2 ground state and suggest a covalent environment with in-plane sigma and pi-bonds provided by the ligand system. The characteristic i.r. data have conjectured that the pyrazolyl nitrogen (tertiary), azomethine nitrogen and the thiolato sulphur atom are coordinated to copper(II) ion; the bands at 420, 320 and 290 cm - 1 assigned to ν C u - N ( a z o m ) , ν C u - S and ν C u - N ( P z ) respectively support the proposition.The X-ray crystal structure (Fig. I) of the complex clearly reveals that copper(II) ion lies in an approximate square planar environment with the schiff base coordinated as uninegatively charged tridentate NNS ligand via the pyrazolyl nitrogen [N(1)] the azomethine nitrogen [N(3)] and the mercaptide sulphur atom [S(1)]; the fourth coordination position around copper(II) is occupied by the chloride ion. The fact that the ligand binds copper(II) as the iminothiolate form is substantiated by the C(5)-S(1) bond length of 1.745(3) 9. The structure significantly shows novel instance of a nucleophilic substitution on the azomethine function; the H-atom on C(4) is replaced by an ethoxy group (coming from the ethanolic solvent) presumably promoted by copper(II).