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Element 105 / Hahnium / Chemical properties / Group 5 elements / Relativistic molecular orbital calculations AbstractTo study the complex formation of group 5 elements (Nb, Ta, Ha, and pseudoanalog Pa) in aqueous HCl solutions of medium and high concentrations the electronic structures of anionic complexes of these elements [MC1 6 ]~, [MOCLJ", [M(OH) 2 Cl 4 ]~, and [MOCI5] 2 " have been calculated using the relativistic DiracSlater Discrete-Variational Method. The charge density distribution analysis has shown that tantalum occupies a specific position in the group and has the highest tendency to form the pure halide complex, [TaCl 6 ] ~. This fact along with a high covalency of this complex explains its good extractability into aliphatic amines. Niobium has equal trends to form pure halide [NbCl 6 ]~ and oxyhalide [NbOCl 5 ] 2 " species at medium and high acid concentrations. Protactinium has a slight preference for the [PaOCl 5 ] 2-form or for the pure halide complexes with coordination number higher than 6 under these conditions. Element 105 at high HCl concentrations will have a preference to form oxyhalide anionic complex [HaOCl 5 ] 2~ rather than [HaCl 6 ] ". For the same sort of anionic oxychloride complexes an estimate has been done of their partition between the organic and aqueous phases in the extraction by aliphatic amines, which shows the following succession of the partition coefficients: Ρ Nb < Phu < ipa·

Relativistic self-consistent charge Dirac–Slater discrete variational method calculations have been done for the series of molecules MBr5, where M=Nb, Ta, Pa, and element 105, Ha. The electronic structure data show that the trends within the group 5 pentabromides resemble those for the corresponding pentaclorides with the latter being more ionic. Estimation of the volatility of group 5 bromides has been done on the basis of the molecular orbital calculations. According to the results of the theoretical interpretation HaBr5 seems to be more volatile than NbBr5 and TaBr5.

Electronic structures of MOCl3 and MOBr3 molecules, where M=V, Nb, Ta, Pa, and element 105, hahnium, have been calculated using the relativistic Dirac–Slater discrete-variational method. The character of bonding has been analyzed using the Mulliken population analysis of the molecular orbitals. It was shown that hahnium oxytrihalides have similar properties to oxytrihalides of Nb and Ta and that hahnium has the highest tendency to form double bond with oxygen. Some peculiarities in the electronic structure of HaOCl3 and HaOBr3 result from relativistic effects. Volatilities of the oxytrihalides in comparison with the corresponding pentahalides were considered using results of the present calculations. Higher ionic character and lower covalency as well as the presence of dipole moments in MOX3 (X=Cl, Br) molecules compared to analogous MX5 ones are the factors contributing to their lower volatilities.

Standard redox potentials Eo(Mz++X/MZ+) in acidic solutions for group 5 elements including element 105 (Ha) and the actinide, Pa, have been estimated on the basis of the ionization potentials calculated via the multiconfiguration Dirac-Fock method. Stability of the pentavalent state was shown to increase along the group from V to Ha, while that of the tetra-and trivalent states decreases in this direction. Our estimates have shown no extra stability of the trivalent state of hahnium. Element 105 should form mixed-valence complexes by analogy with Nb due to the similar values of their potentials Eo(M3+/M2+).The stability of the maximum oxidation state of the elements decreases in the direction 103 > 104 > 105.

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