Probing the strong magnetic exchange behaviour of transition metal-radical complexes: a DFT case study

Attaining very large magnetic exchange coupling values for polymetallic clusters is of great interest in the area of Single-Molecule Magnets (SMMs). Direct contact of paramagnetic centers in metal-radical complexes is known to yield very large magnetic exchange coupling values, but the estimation of such magnetic coupling values for polymetallic metal-radical complexes is not straightforward. Here, five previously reported M2-radical complexes (where M = CrIII, MnII, FeII, and CoII) have been studied using density functional theory. Our calculations suggest that a very strong overlap between the metal 3d orbitals and the radical's singly occupied π* orbital is the foremost reason for reported large magnetic exchange values. Our used methodology also gives a precise estimate of magnetic coupling values for this class of complexes. Model calculations and magneto-structural correlation studies suggest the importance of structural parameters, such as the metal-N(radical) distance and metal-radical planarity, in attaining very higher magnetic exchange coupling values with small structural modification.