Antiferromagnetic Coupling Supported by Metallophilic Interactions: Theoretical View

The antiferromagnetic coupling supported by metallophilic interactions has been studied in the framework of the broken symmetry approach (BS) and multiconfigurational calculations (CASSCF). A series of heterobimetallic complexes of the form [PtCo(X)₄(Y)]₂ (X = tba thiobenzoate, SAc thioacetate, and Y = H₂O, NO₂py, py), previously reported, have been used as model systems. Magnetic coupling constants were found in good agreement with the experimental reports, and it could be concluded that axial ligands with a pure σ-donor character have a marked effect on the J value strengthening the antiferromagnetic coupling, as shown for [PtCo(SAc)₄(H₂O)]₂ and [PtNi(SAc)₄(H₂O)]₂. The latter complex, included for comparative purposes, also made it possible to evidence that the interaction between magnetic orbitals and low-level excitation in the Pt···Pt region is also relevant favoring the stronger antiferromagnetic coupling found in this case. A careful analysis of the energetic components involved in Pt···Pt interaction suggests that the stabilization arises from a combination of favorable orbital contributions, which allows a weak covalent Pt···Pt σ(dz²...dz²) bond. Theoretical tools evidence that the weak σ-bond found between monomeric units is responsible for a spin polarization mechanism resulting in the observed antiferromagnetic interaction. Multiconfigurational calculations finally allowed us to establish that the spin polarization mechanism involves not only the d orbitals in the M-Pt···Pt-M bond direction but also the empty 6p orbitals of Pt atoms. The inclusion of these orbitals favors a correlation-induced delocalization of magnetic orbitals and therefore a better balance among direct and kinetic exchange. The results shown in this work are relevant in the molecular design of systems supported by metallophilic interactions not only between platinum atoms but also could be extended to other cases with similar interactions.