The actinyl ions, consisting of the dioxoactinde(VI) and dioxoactinide(V) ions, are unique in the periodic table. However, very few quantitative studies of the spectra of these ions have been performed. In this paper the analysis of the optical spectrum of the aqueous neptunyl(V) [NpO2(+)] ion in 1 M HClO4 is reexamined. The species in solution is assumed to be the linear NpO2(+) ion surrounded in the equatorial plane by five oxygen atoms from five H2O molecules. The neptunyl(V) ion has the 5f(2) open-shell configuration, and the low-lying optical transitions (up to ∼20 000 cm(-1)) observed are primarily from the two 5f electrons occupying the 5fφ and 5fδ orbitals. The conventional parametric theory used for f(n) systems is applied to these low-lying transitions utilizing the intensity calculations that Matiska et al.1 have performed for this ion and the data reported by Eisenstein and Pryce.2 Possible ratios for the Slater electrostatic repulsion parameters are obtained from the data for the isoelectronic ion U(4+) (5f(2)) in various host crystals. The results are consistent with earlier crystal field analyses of the 5f(1) neptunyl(VI) [NpO2(2+)] ion.

The electronic structure of Th(Cp″)3 (Cp″ = η5-C5H3(SiMe3-1,3), a molecule which has a 6d1 ground configuration and an excited 5f1 configuration beginning approximately 14,000 cm−1 higher, is reexamined. The ground state of the 6d1 configuration in a crystal field of D3h symmetry is shown to be an orbitally quenched 2A1ʹ state, best described as a pure 2D1/2 state. Electric dipole selection rules from this state to the states for the 5f1 configuration in a D3h crystal field result in the 5f1 truncated optical spectrum observed. The optical spectrum of the square planar Th3+ compound [Li(THF)4][Th(OAr′)4], where (OAr′ = OC6 H2tBu2-2,6-Me-4), is discussed.