Four new 5-formylpyrazole-based thiolato Schiff base nickel(II) complexes (1-4) have been synthesized from the corresponding ligands and nickel(II) acetate. A new 2-mercaptobenzaldehyde Schiff base nickel(II) complex (5) has been synthesized by two routes. X-ray structural analyses on three of the complexes showed the following : 1, chemical formula C26H26N6S2Ni, monoclinic, space group P2(1)/n with a = 10.438(2) Angstrom, b = 12.450(3) Angstrom, c = 19.616(3) Angstrom, beta = 94.33(1)degrees, Z = 4, and R = 0.044 for 3365 reflections; 4, chemical formula C34H26N6S2Ni, monoclinic, space group C2/c with a = 19.242(3) Angstrom, b = 9.757(2) Angstrom, c = 19.691(3) Angstrom, beta = 125.84(1)degrees, Z = 4, and R = 0.032 for 2206 reflections; 5, chemical formula C28H22N2S2Ni, monoclinic, space group C2/c with a = 25.230(4) Angstrom, b = 11.098(2) Angstrom, c = 17.287(8) Angstrom, beta = 98.46(3)degrees, Z = 8, and R = 0.047 for 1912 reflections. The coordination geometry in 1 is distorted tetrahedral while in 4 and 5 it is close to cis-square planar despite the influence of the (6,6’-dimethyl)biphenyl group in the tetradentate ligand backbone. The bond distances are as follows. Ni-S : 1, 2.239(1), 2.243(1) Angstrom; 4, 2.185(1) Angstrom; 5, 2.152(2), 2.184 (2) Angstrom. Ni-N : 1, 1.963(2), 1.968(2) Angstrom; 4, 1.927(2); 5, 1.931(5), 1.917(6) Angstrom. The distortions from tetrahedral to planar geometry are manifest in some interesting changes in dihedral angles and donor atom "bite" distances. The magnetic susceptibilities were measured in the temperature range 4.2-300 K for 1-5. The susceptibilities of 1 and 2 could be fitted well to a simple axial spin Hamiltonian with S = 1, D = 34 cm(-1) and S = 1, D = 53 cm(-1), respectively. Complexes 3-5 are diamagnetic and showed only small temperature-independent paramagnetic susceptibilities. Although no temperature-induced spin changes were observed for solids of 1-4, they all showed a (1)A(lg) reversible arrow T-3(1) equilibrium in chloroform solutions. Each of complexes 1-5 show LF, CT, and intraligand spectral features in the spectral region 5,000 - 40,000 cm(-1). Bands assigned to the high-spin and low-spin forms are observed separately. The thermodynamic parameters for the high-spin reversible arrow low-spin equilibria in chloroform solutions were obtained by using the Evans method, variable-temperature H-1 NMR spectra, and variable-temperature electronic spectra. Complexes 1-4 exhibit equilibrium constants K-eq (=N-HS/N-LS) in the range of 0.456-0.080.