Tetrahedral and Square Planar Ni[(SPR₂)₂N]₂ complexes, R = Ph & ⁱPr Revisited: Experimental and Theoretical Analysis of Interconversion Pathways, Structural Preferences, and Spin Delocalization

Sulfur-containing mono- or bidentate types of ligands, usually form square planar Ni(II)S4 complexes. However, it has already been established that the bidentate L− dithioimidodiphosphinato ligands, [R2P(S)NP(S)R′2]−, R, and R′ = aryl or alkyl, can afford both tetrahedral and square planar, NiS4-containing, homoleptic NiR,R′L2 complexes, owing to an apparent structural flexibility, which has not, so far, been probed. In this work, the literature tetrahedral Ni[R2P(S)NP(S)R2]2 complexes, R = Ph (NiPh,PhL2, 1Td) and R = iPr (NiiPr,iPrL2, 2) as well as the newly synthesized Ni[iPr2P(S)NP(S)Ph2]2 complex (NiiPr,PhL2, 3), have been studied by UV−vis, IR, and 31P NMR spectroscopy. Complex 3 was shown by X-ray crystallography to be square planar, and magnetic studies confirmed that it is diamagnetic in the solid state. However, it becomes paramagnetic in solution, as it shows a similar UV−vis spectrum to one of the tetrahedral 1Td and 2 complexes. The crystal structure of the potassium salt of the asymmetric ligand, [iPr2P(S)NP(S)Ph2]K, has also been determined and compared to those of the protonated iPr2P(S)NHP(S)Ph2 ligand and complex 3. All three, 1Td, 2, and 3, NiR,R′L2 complexes show strong paramagnetic effects in their solution 31P NMR spectra. The magnetic properties of paramagnetic complexes 1 and 2 in the solid state were investigated on oriented crystals, and their analysis afforded remarkably small values of the spin−orbit coupling constant (λ) and orbital reduction factor (k) parameters, implying significant delocalization of unpaired electronic density toward the ligands. The above experimental findings are combined with data from standard density functional theory and correlated multiconfiguration ab initio theoretical methods, in an effort to investigate the interplay between the square planar and tetrahedral geometries of the NiS4 core, the mechanistic pathway for the spin-state interconversion, the degree of covalency of the Ni−S bonds, and the distribution of the spin density in this type of system. The analysis provides justification for the structural flexibility of such ligands, affording NiR,R′L2 complexes with variable metallacycle conformation and NiS4 core geometries. Of particular importance are the large zero-field splitting values estimated by both experimental and theoretical means, which have not, as yet, been verified by direct methods, such as electron paramagnetic resonance spectroscopy. The findings of our work confirm earlier observations on the feasibility of synthesizing either tetrahedral or square planar NiS4 complexes containing the same type of ligands. They can also form the basis of investigating structure−properties relationships in other NiS4-containing systems.