Electron Spin Resonance in Transition Metal Chelates. I. Copper (II) Bis-Acetylacetonate

Electron spin resonance at 3.2 cm has been observed from copper(II) in single crystals of copper(II) bis‐acetylacetonate diluted to ½ mole % with palladium(II) bis‐acetylacetonate at 77°K and 300°K. The parameters of the spin Hamiltonian have been expressed in terms of the atomic orbital coefficients of molecular orbital wave functions describing the electron hole in the orbital ground state and two excited states. Experimental determination of the spin Hamiltonian then results in expressions for the molecular orbital wave functions. The results indicate that the in‐plane sigma and pi bonding are appreciably covalent, whereas the out‐of‐plane pi bonding with the acetylacetonate ring is ionic. The hyperfine interaction may be expressed by a relation of the type $K^2{g}^2{\mathrm{=A}}^2{g}_{\parallel }^2{\cos }^2{\mathrm{\theta +B}}^2{g}_{\perp }^2{\sin }^2{\text{θ+2C}}^2{g}_{\parallel }{g}_{\perp }\sin \theta \cos \theta$ , where C² increases with decreasing temperature. The appearance of the term in $\sin \mathrm{\theta ·}\cos \theta$ is attributed to low symmetry crystal fields due to the crystallographic array, the main symmetry being determined by the molecule itself. A small amount of rhombic character was observed in the g tensor, but was not enough to make an accurate location of the g_x and g_y directions possible. For the purpose of analysis, the g factor is expressed in terms of the two principal values g∥ and g_⊥ which do not vary between 77°K and 300°K to the accuracy of the determination.