Trivalent Metal Complexes of Pentadentate Ligands

Department of Chemistry, D. N. College, Meerut-250 002. (India) The complexes of trivalent metals-chromium, manganese, Iron and cobalt of two pentadentate ligands,2,6-dipicolinic acid hydrazide (DPH) and N,N'·dibenzylidene dipicolinic acid hydrazide (DBPH) have been described. These complexes have been characterised by their elemental analysis and by molar conductance, electronic,lnfrared spectral (conventional and far i r.) and magnetic susceptibilities measurements. Two types of complexes are isolated, (a) the first type Includes the complexes of DPH which may be formulated as [M(DPH)X] X2 , and (b) the complexes of DBPH which have a general formula of the type [M(DBPH)]X2 where M = Cr(III), Mn(III), Fe(lll) or Co(III), X= Cl, NO3 or OAc. In the case of cobalt complexes X of coordination sphere is OH. The first type of complexes are six-coordinate, while the complexes of DBPH are five-coordinate, as it has been shown by infrared spectra that both the ligands, DPH and DBPH. act as pentadentate, having coordination sites at pyridine-nitrogen, amide-oxygen and hydrazinic nitrogen atoms. The quinquedentate nature of the ligand is confirmed by far infrared spectra where characteristic bands of v(M-py), v(M-N) and v(M-O) vibrations are assigned. The various ligand field parameters (Dq, Ds, and Dt) have been calculated in conjunction with Racah interelectronic repulsion parameters B and C. In the case of chromium and manganese complexes, the use of Normalised Spherical Harmonic (NSH) Hamiltonian theory has been made to evaluate various parameters DQ, DS and DT. The degree of distortion of chromium and manganese complexes calculated in terms of DT/DQ lie In the range 0.17-0.18 and 0.34, respectively, indicating that in chromium complexes the metal atom is slightly above the basal plane of the square pyramid, while in the case of manganese complexes the displacement of metal is appreciable. The severe distorted nature of manganese complexes is probably due to the operation of Jahn-Teller effect, in addition to inequivalence of the donor atoms present in the ligands