Estimation of covalent effects in neptunium compound on the basis of moessbauer isomer shifts

A model to calculate covalent effects in neptunium compounds with different ligands is suggested. A correlation between population of 5f-orbitals of neptunium and Moessbauer isomer shifts on 237Np nuclei is made. It is shown that in actinide fluorides AnFx (An=U, Np, Pu and Am, x=3-6) covalent effects increase with an increase in oxidation state and decrease with an increase in actinide ordinal number, i. e. from U to Am. In neptunates populations of 5f-orbitals decrease, and those of 6d-orbitals increase with an increase in oxidation state, which corresponds to a higher delocalization of electrons and enhancing effects of (f-d)-hybridization. In neptunates (6) and (7) the higher is the deviation from octahedral symmetry, the greater are covalent effects. In compounds with neptunium-type bond the vacancy on the neptunyl bond in compound NaNp1.5O3 in terms of oxygen decreases its multiplicity and increases ionic character. On the whole, compounds with neptunyl bond are more covalent than neptunates. When compairing isovalent pairs NpO6z- and NpFx, the inversion of ionicity is observed: oxygen compounds are more ionic than fluorides in lower oxidation states, and fluoride compounds - than fluorides in higher oxidation states