Inspection of the duality of a verdazyl-based radical in transition metal complexes: a pi* donor ligand and a magnetic partner.

The behavior of a verdazyl-based radical bound to open-shell transition metal ions in the structurally and magnetically characterized [M(hfac)2imvd(o)] (M = Mn, Ni; hfac = (1,1,1,5,5,5)hexafluoroacetylacetonate; imvd(o) = 3-(2'-imidazolyl)-1,5-dimethyl-6-oxoverdazyl) complexes is rationalized using ab initio wave-function-based calculations analysis. The calculated exchange coupling constants J (H = -J(s(M) x s(imvd(o)); J(Mn)(calcd) = -63 cm(-1), J(Ni)(calcd) = 205 cm(-1)) are in excellent agreement with the experimental ones (J(Mn)(exp) = -63 cm(-1), J(Ni)(exp) = 193 cm(-1)). Even though both rings are involved through the binding mode of the imvd(o) radical, the spin density remains essentially localized on the nitrogen-rich ring. The singularity stems from its bidentate coordinating character. The analysis of the correlated wave function suggests that the verdazyl-based radical acts as a pi* donor ligand which allows ligand-to-metal charge transfer and excludes metal-to-ligand charge transfer. This reflects the weak covalent character of the M-imvd(o) pi coordination bond. From a magnetic point of view, the through-space exchange governs the ferromagnetic character in the Ni derivative up to 153 cm(-1) as expected from a description limited to the magnetic orbitals. Nevertheless, the CI expansion displays the participation of excited doublet and quartet states (spin polarization) on the verdazyl moiety which leads to a significant additional ferromagnetic contribution (52 cm(-1)). In the [Mn(hfac)2imvd(o)] analogue, the antiferromagnetic contribution arising from kinetic exchange is only one-third of the observed exchange coupling constant. It is necessary to introduce dynamical correlation effects to quantitatively recover the exchange interaction in this compound. Since the pi* donor and spin-polarized characters of the verdazyl moiety dominate over the negligible polarizability of the imidazole part, it is concluded that the noninnocent nature of the imvd(o) radical is held by the verdazyl ring part.