In this work, a new family of binuclear Ni^II–Ln^III complexes with the formula [NiLn(L)₂(NO₃)₃]·0.5H₂O (Ln = Gd, 1; Tb, 2; Dy, 3; Ho, 4; Er, 5; Yb, 6; Y, 7) was synthesized using a thioether group-bearing Schiff base. Due to the strict hard/soft dichotomy between the 4f and 3d metal ions, selective coordination of Ni^II and 4f metal ions was achieved with the adjacent soft ONS and hard OO binding pockets of the ligand. All the complexes 1–7 exhibit a Ni^II centre in a distorted pseudo-octahedral geometry with the Ln^III centres in distorted bicapped square-antiprism geometry. The huge distortion around the Ni^II centres is triggered for the accommodation of larger lanthanoids to the adjacent OO coordination site, and this forces the Ni^II centres to have a tridentate coordination from the ONS, as intermediate between meridional and facial binding. Field-induced single-molecule magnetic behaviour was observed for heterodinuclear complexes involving Kramers lanthanide ions (Ln^III = Dy, Er and Yb), with magnetic relaxation occurring through an Orbach process only for 5. DFT calculations using various functionals (BP86, B3LYP, PBE0, TPSSh, PWPB95, R²SCAN) were applied to calculate the isotropic exchange, showing good agreement with the experiment (J_Gd–Ni = +1.78 cm⁻¹). CASSCF calculations for Ni^II and Ln^III ions were also performed to reveal detailed information about their electronic structure and magnetic anisotropy, supporting the experimental observations. This study accentuates the mutual distortion of coordination geometry induced by flexibility of the ligand backbone with the simultaneous binding of two different metal ions.