New cobalt(II) and nickel(II) complexes with 2-(pyrazol-1-ylmethyl)pyridine. Stereochemical variations in cobalt(II) complexes and X-ray crystal structure of bis[2-(pyrazol-1-ylmethyl) pyridine]dichlorocobalt(II) tetrahydrate

Abstract

Using a non-planar bidentate ligand 2-(pyrazol-1-ylmethyl)pyridine (L₄) a group of three high-spin cobalt(II) (S = 3/2) and two paramagnetic nickel(II) (S = 1) complexes of compositions [CoL⁴Cl₂] 1, [ML₂⁴Cl₂]·4H₂O (M = Co, 2; M = Ni, 4) and [ML₃⁴] (ClO₄)₂·H₂O (M = Co, 3; M = Ni, 5) have been prepared and their stereochemical properties investigated. Characterizations of the new complexes have been done be elemental analysis, conductivity measurements, solution-state magnetic moment, infrared and absorption spectral data. A representative six-co-ordinate complex of cobalt(II), 2 has been structurally characterized. The pertinent bond lengths (Å) are: Co---Cl, 2.413(4) (Å) Co---N (pyridine), 2.198(6) (Å) Co---N (pyrazole), 2.106(7) (Å) Variable-temperature (80-300 K) magnetic susceptibility measurements on solid samples of 2 and 3 confirm their high spin character (S = 3/2). The blue or pink cobalt(II) and light green or violet nickel(II) complexes display ligand field transitions. For 3 and 5, analyses of their d-d transition energies reveal that L⁴ is a weak field ligand, comparable to that of L³ [2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine]. For cobalt(II) complexes in the solution state stereochemical interconversions have been neatly demonstrated.