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.
Hydrogen bonding to metals as a probe for an inverted ligand field
