Reaction of the deprotonated N-thiophosphorylated thioureas RC(S)NHP(S)(OiPr) 2 [R = EtNH (HL I ), iPrNH (HL II ), Et 2 N (HL III ), 2,5-Me 2 C 6 H 3 NH (HL IV ), 4-Me 2 NC 6 H 4 NH (HL V )] with Ni II leads to complexes of the formula [NiL I-V 2 ]. The molecular structures of the complexes in the solid were elucidated by single-crystal X-ray diffraction analysis. In the complexes, the metal atom is found to be in a square-planar trans-N 2 S 2 ([NiL II,IV 2 ]) environment formed by the C=S sulfur atoms and the P-N nitrogen atoms, or in a square-planar trans-S 2 SЈ 2 ([NiL I,III 2 ]) environment formed by the C=S and P=S sulfur atoms of two deprotonated ligands. Reaction of deprotonated N-thiophosphorylated thiourea HL V with NiCl 2 leads to violet [Ni(L-1,3-N,S) 2 ] or dark violet [Ni(L-1,5-S,SЈ) 2 ]· (CH 3 ) 2 C=O crystals that were isolated by recrystallization from a mixture of CH 2 Cl 2 or acetone, respectively, and nhexane. DFT calculations confirmed that the [Ni(L I,II,IV,V -[a] Institute of Condensed Matter and Nanosciences, 545 N,S) 2 ] conformers are more stable (by 5-7 kcal/mol) than [Ni-(L I,II,IV,V -S,SЈ) 2 ], whereas [Ni(L III -N,S) 2 ] is less stable (by 7-9 kcal/mol) than [Ni(L III -S,SЈ) 2 ]. The main reason for higher stability of the 1,3-N,S versus 1,5-S,SЈ isomers is the formation of intramolecular N-H···S=P hydrogen bonds. The same hydrogen bonds are impossible in complex [NiL III 2 ]. In solution, complex [NiL III 2 ] has revealed an exclusively 1,5-S,SЈ coordination, whereas compounds [NiL I,II,IV,V 2 ] reveal at least two isomers in the 1 H and 31 P{ 1 H} NMR spectra. The major species is assigned to the 1,3-N,S-coordinated isomer, and the minor signals are assigned to the 1,5-S,SЈ isomer, which was confirmed by UV/Vis spectroscopic results. The electrochemical measurements reveal reversible one-electron reduction and irreversible oxidations both assigned to ligand-centred processes. Ligand-based oxidation processes agree well with TD-DFT results.Eur. J. Inorg. Chem. 2013, 545-555 [NiL I,II,IV,V 2 ] contain the characteristic band for the alk-ylNH or arylNH group at 3138-3369 cm -1 . Furthermore, the spectrum of the acetone solvate [Ni(L V -1,5-S,SЈ) 2 ]· (CH 3 ) 2 C=O contains an intense band at 1702 cm -1 corresponding to the C=O group.The 31 P{ 1 H} NMR spectra of [NiL I,II,IV,V 2 ] in CDCl 3 are rather complicated and contain two signals at δ = 50.7-50.9 and 53.6-54.3 ppm for [NiL I,IV 2 ] and four signals at δ = 50.7-51.0, 53.5-54.4, 55.0-55.3 and 58.0-58.4 ppm for [NiL II,V 2 ]. The same was previously observed in the spectra of the complexes [Ni{RNHC(S)NP(S)(OiPr) 2 } 2 ] (R = tBu, Ad, Ph, 2-MeC 6 H 4 , 2,6-Me 2 C 6 H 3 , 2,4,6-Me 3 C 6 H 2 ) in CD 2 Cl 2 or CDCl 3 . [4d,8,10] The high-field signal corresponds to the trans-1,5-S,SЈ coordination, whereas the second signal is typical for the trans-1,3-N,S coordination of the deprotonated ligands. [4d,8,10] In view of the similar intensity and line width of the two low-field signals in the spectra of [NiL II,V 2 ], they are diastereot...