ESR and Optical Absorption Studies of the Copper(II) Complexes of Ethylenediamine and Its Alkyl Derivatives

The ESR and the optical absorption of the copper(II) complexes of ethylenediamine and a series of alkylated ethylenediamines have been investigated in order to estimate the character of the chemical bonding in the copper(II) complex. The covalency of the bonding of the copper atom to the ligand nitrogen atoms in the complexes has been discussed in connection with the chemical and physical properties of the complexes or of their ligand molecules themselves. The ESR results indicate that the σ bonding of the complexes becomes more covalent as the number of alkyl groups in their ligand molecules increases; furthermore, the tendency toward the σ bonding becomes more pronounced when the alkyl groups are ethyl rather than methyl and when they are N-substituted alkyl groups rather than C-substituted ones. It is also indicated that there is a linear relationship between the energies of the ultraviolet absorption band for the complexes and the relative ionization potentials estimated for their ligand molecules, and that absorption band can be assigned to an electron transfer from a ligand σ orbital to an antibonding orbital associated with the copper atom. From these results, it may be concluded that, as the σ bonding becomes more covalent, this charge transfer band shifts to a longer wavelength.