Theoretical Investigations Of Oxidation State And Periodicity In Transition Metal Compounds

Oxidation state of elements has been developed to be a central concept in chemistry,since Lavoisier first introduced the concept of oxidation in the late 18 th century.However,the exact definition of oxidation state concept is still vague,whilepeople usually assume that the oxidation state of oxygenis-2.However,the oxidation state of transition-metal elements is complicated,the simple rulecannot simply be applied to some compounds.Therefore,the application of some physical parameters should be useful in determining the actual oxidation state.In some transition metal clusters there might bemetals with a variety of oxidation states,which makes the ground state of transition metal clusters hard to be determined.We mainly focused on determining the maximum oxidation states for the group-8 transition metal tetroxide,because the group-8 transition metal elements might have ? oxidation state that is found to be the highest oxidation state of neutral compounds so far for the whole periodic table.It was also of interest to investigate the appropriate methods for these considered compounds,the reasons for forming certain oxidation state,andthe principles of the changes in the oxidation state.We calculated various electronic states of FeO₄ systems and analyzed the oxidation states for Fe.By extensive theoretical analyses,it is found that the ground state ¹C_2v-[Fe?O₂]²⁺?O₂?^2-and a meta-stableFe?VIII?state of FeO₄ are highly competitive in energy.Density functional approaches?DFA?and some single-reference methods of wavefunction theory?WFT?resulted in up to 5eV deviation to the highly accurate multi-reference WFT calculations.The density-matrix renormalization group?DMRG?method should be suitable to be applied on FeO₄ system with comparable results to the accurate multi-reference calculations,and can be used to examine newtheoretical methods developed for multi-reference systems in the future.Although Pu has eight valence electrons as Ru and Os that can form stable compounds such as RuO₄ and OsO₄,it is difficult to form stable Pu?VIII?compounds.As PuO_nF_8-2n?n = 0-4?molecules arethe most likely ones to form Pu???,weapplied a variety of methods to establish that the ground state is⁵C_2v-?PuO₂?⁺?O₂?^-.Pu?V?with a?f3?electronic stateforms a bent plutonyl with two oxygen atoms,which is then connected by a superoxideligand to form the ground state PuO₄ compound.In the other systems,two oxygen atoms will also form a superoxide whenthe number of oxygen atoms in the molecule is more than two.The 1Oh PuF₈ species is found to decompose into PuF₆ and F₂ molecules,and¹C_2v PuOF₆ also decomposes into PuF₆ and 1 / 2O₂ molecules.Therefore,it is difficult to form stable Pu?VIII?neutral compoundsunder normal conditions.It is of interest to extend the theoretical study to include all MO₄?M= Fe,Ru,Os,Hs,Sm,Pu?molecules to understand why higher than ? oxidation state is difficult to achieve.We studied all the metal atoms M in MO₄ of the group ? elementswith eight valence electrons.It is found that the heavy d-block elements?Ru,Os,and Hs?can form a stable octavalent compound,M+8?O-2?4.However,the 3d-,4f-and 5f-elements tend to form relatively lower oxidation states,with Fe+6,Sm+3,Pu+5,respectively.We have shown that the valence orbital energy and radius of the metal elements play important roles in determining the oxidation state,which is further verified by the results from theoretical studies on MS₄?M = Fe,Ru,Os,Hs,Sm,Pu?molecules.We have also found that the utilization of the difference of electronegativity,ionization energy,and radial distribution of different valence orbitals can help to explain the high oxidation states.Understanding the trend of these oxidation stateslays a foundation for designing transition metal compounds with unusual oxidation states.