Shedding light on main-group dithiolene chemistry: electronic and geometrical perspectives of tris(dmit) complexes

1,3-Dithiola-2-thiona-4,5-dithiolate is a versatile noninnocent ligand with applications in superconductivity, magnetism, and nonlinear optical materials. This study evaluated the tris(dmit) antimony(V) and tin(IV) complexes via modern computational methods. A local energy decomposition analysis of metal‒sulfur bond formation revealed that the distorted geometry of the tris(dmit) complexes in acetonitrile is the most stable conformation for both systems, whereas other conformations remain energetically accessible. The geometrical stability arises from the ionic and soft acid‒base interactions between the highly oxidized cations and thiolated sulfur atoms. State-averaged complete active-space self-consistent field with N-electron valence second-order perturbation theory correction calculations indicated that while the ground states are dominated by a single configuration, the excited state manifold in both systems shows multiconfigurational character, which is relevant for understanding systems with potentially non-innocent ligands. Finally, similarity-transformed equations of motion coupled-cluster calculations successfully reproduced the experimental UV‒Vis spectra of the two complexes in acetonitrile, highlighting the low-energy ligand-to-metal charge-transfer excitations in the tris(dmit) antimony(V) complex. These findings increase the understanding of the electronic structure and stability of tris(dmit) complexes, which can help in understanding potential applications.

The tris(dmit) complexes were computationally investigated via two solvation models: implicit and explicit solvation. All ab initio and DFT wave function calculations were performed via ORCA software version 5.0.3. Model implicit solvation were optimized via the TPSSh/Def2-TZVP level of theory with CPCM used to simulate an acetonitrile medium. AIMD calculations for explicit solvation of the dmit salts were conducted using the GFN2-xTB method with 40 explicit acetonitrile molecules as the solvent at 300 K for a total simulation time of 35.0 ps, a timestep of 0.2 fs and data dumps every 10.0 fs. The final geometries were optimized via an ONIOM approach, with the high-level region set at the R2SCAN-3C method, which included the complexes and the first solvation shell. The low-level region utilized the extended tight-binding (xTB) method to encapsulate the explicitly solvated models, which comprised the remaining solvent molecules. Local energy decomposition (LED) analysis at the DLPNO-CCSD(T)/Def2-TZVP level of theory was utilized to investigate the stability of the complex geometries identified by AIMD. The electronic structures of the complexes were assessed using the SA-CASSCF/NEVPT2/Def2-TZVP method to confirm the multiconfigurational and multireference nature of their electronic structures. Electronic spectra were analyzed using the STEOM-DLPNO-CCSD/Def2-TZVP method, with CPCM used to simulate an acetonitrile medium.