Optical rotatory power of co-ordination compounds. Part XVIII. The circular dichroism of trigonal nickel(II) chelate complexes

Abstract

The axial circular dichroism spectrum of [Ni(en)₃]²⁺ in the hexagonal host crystal, [Zn(en)₃](NO₃)₂,6H₂O, has been measured, together with the solution c.d. spectra of tris-1,2-diamine and tris-1,2-di-imine complexes of nickel(II). The results show that the spin-allowed octahedral transition of nickel(II) with the lowest energy, ³A_2g→³T_2g, is allowed for magnetic dipole radiation and breaks down in tris-1,2-diamine complexes with the Δ-configuration into a lower-energy doubly-degenerate component with a negative rotational strength and a higher-energy non-degenerate component with a positive rotational strength, like the corresponding transition in the analogous complexes of d³ and low-spin d⁶ transition-metal ions. The similarities between the d³, d⁶, and d⁸ cases are found to extend to the c.d. absorption of the second d-electron transition and of the u.v. charge-transfer transition. The complex [Ni(tren-py₃)]²⁺{tren-py₃= tris-[4-(2-pyridyl)-3-azabut-3-enyl]amine} has been resolved and the Δ-configuration is tentatively assigned to the (–)-isomer from its c.d. spectrum.