Metal(II) bis-chelates of
the type ML2 [M = CoII, NiII, CuII;
L = 2'- hydroxy-5'-X-chalcone where X = H, CH3, Cl]
have been prepared and studied. Structures have been assessed by the
measurement of magnetic moments, ligand field and infrared spectra and thermal
properties. These chelates possess low-spin trans-square-planar configuration
and show resistance to adduct formation in contrast to metal(II) chelates of
β-diketones, salicylaldehyde,
o-hydroxyaryl ketones and esters and o-hydroxy-crotonophenones, which have high-spin octahedral
configuration. Extensive conjugation lowers the energy of the π3*
orbital which enters into a very strong dπ-π3*
interaction leading to spin-pairing. ��� Infrared spectra indicate that the carbonyl
group is perturbed only slightly by coordination to metal. A change in metal
ion affects v(C=O), v(M-O) and other vibrations and the order of stability,
namely, Co ≈ Ni < Cu, inferred from these vibrations is as expected
for the low-spin square configuration of the chelates. Introduction of substituents
(5'-X) alters only v(M-O)
significantly and the order of stability, namely, Cl
> CH3 > H, derived from v(M-O) is consistent with Taft's resonance
polar parameters of the substituents. All these observations are explained by
the electron sink property of the phenyl group.
Correlating Axial and Equatorial Ligand Field Effects to the Single-Molecule Magnet Performances of a Family of Dysprosium Bis-Methanediide Complexes