Cobalt(II), nickel(II) and copper(II) complexes of some o-hydroxycrotonophenones

Abstract

Metal(II) complexes of the type ML₂,nB [M = Cu^II, Ni^II, Co^II; L = 2- hydroxy-5-X-crotonophenone where X = H, CH₃, Cl; B = H₂O, pyridine; n = 0, 1, 2] have been obtained and investigated. With the help of element analyses, magnetic measurements, ligand field and infrared spectra and thermal studies, the structure and the nature of bonding have been established. The anhydrous copper(II) chelates are monomeric and possess trans-square-planar configuration while the corresponding cobalt(II) and nickel(II) compounds are polymeric and possess high-spin trans-octahedral configuration. All the base adducts possess high-spin trans-octahedral structure with lesser tendency toward dissociation in solution. Infrared studies indicate that v(C=O) and v(M-O) are affected by metal ion and phenyl substitutions and adduct formation. The order of stabilities, namely Cu > Ni > Co, derived from v(M-O) parallels the crystal field stabilization energies. Substitution in the phenyl ring of the complexes produces shifts in v(M-O) which are related to the resonance capacities of the substituents.    

The relatively high ligand field strength of o-hydroxycrotonophenone compared to salicylaldehyde is attributed to the conjugation of C=O with C=C which lowers the energy of the π₃* orbital leading to extensive back-bonding with d_π orbitals of the metal.