Contrary to general belief, the complexes of nickel (II) and tetrahedral structure are rather uncommon. Many compounds which have been attributed such structure have been shown by more detailed investigations to have other stereochemistries. The rare occurrence of such a type of complex, surprising on the basis of Valence Bond Theory, finds a satisfactory explanation when the ⪡ligand field⪢ treatment is applied to the nickel (II) ion. Jensen has shown that while the complexes ( Et 3 P) 2 NiX 2, ( X = Cl, Br, and I) are diamagnetic and have trans-square planar structure, ( Et 3 P) 2 Ni ( NO 3) 2 is paramagnetic (μ eff = 3.10 B. M.) and possibly tetrahedral. It was therefore of interest to study the structure and properties of the compounds ( Ph 3 P 2 NiX 2, ( X = Cl, Br, I, and NO 3). These compounds are paramagnetic (μ eff. = 2.92 − 2.97 B. M.), and have tetrahedral structure as shown by their conductance, electric dipole moment, and X-ray diffraction studies. The thiocyanato analogue ( X = SCN) instead, is diamagnetic and appears to have trans-square planar configuration. The transition from paramagnetic to diamagnetic complexes is attributed to the force of the electrostatic field generated by the ligands, and the formation of tetrahedral compounds, instead of octahedral ones, in the ( Ph 3 P) 2 NiX 2 ( X = Cl, Br, and I), apparently in disagreement with the predictions based on ⪡ligand field⪢ theory, can be explained in terms of steric hindrance, which prevents the formation of octahedral complexes. In general, it appears that tetrahedral complexes of nickel (II) will be formed only when the ligands do not have enough perturbing power to give compounds of the ⪡spin paired⪢ type, and when tetrahedral stereochemistry is forced by the steric requirements of the ligands.