Structural Properties of M(dmit)2-Based (M = Ni, Pd, Pt; dmit2- = 2-Thioxo-1,3-dithiole-4,5-dithiolato) Molecular Metals. Insights from Density Functional Calculations

The bonding between two M(dmit)2 monomers (M = Ni, Pd, Pt) in the stacks that are characteristic for the crystal structure of these compounds has been analyzed, using decomposition of the interaction energy in steric repulsion, electron pair bond formation, and donor−acceptor interactions. Atom over atom (eclipsed) stacking may occur if two conditions are met:  (a) The HOMO to LUMO excitation energy should be small enough so that these singly occupied orbitals may both form an electron pair bond with their partner on the adjacent monomer. The HOMO and LUMO both being ligand based, the electron pair bonds are between the ligands and not between the metals. (b) The bending away of the ligand systems so as to relieve the steric hindrance should not cost too much energy. These circumstances prevail in Pt(dmit)2, where relativistic effects increase the stability of the eclipsed conformation by enhancing the acceptor capability of the virtual orbitals with 6s, 7s, and 6pz character on the metal for electron donation out of the occupied adjacent dz2. In Ni(dmit)2 both the excitation energy is somewhat higher and the bending is more unfavorable; consequently, it does not exhibit the eclipsed stacking pattern. Slipping is another way to relieve the steric repulsion between adjacent monomers. The electron pair bonds are broken in that case, but sufficient donor−acceptor interactions between the two ligand systems remain to make this a viable alternative for Ni(dmit)2. Pd(dmit)2 is in between and may adopt either eclipsed or slipped configurations.