Article pubs.acs.org/IC
On the Highest Oxidation States of Metal Elements in MO4 Molecules (M = Fe, Ru, Os, Hs, Sm, and Pu) Wei Huang, Wen-Hua Xu, W. H. E. Schwarz,† and Jun Li* Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of the Ministry of Education, Tsinghua University, Beijing 100084, China S Supporting Information *
ABSTRACT: Metal tetraoxygen molecules (MO4, M = Fe, Ru, Os, Hs, Sm, Pu) of all metal atoms M with eight valence electrons are theoretically studied using density functional and correlated wave function approaches. The heavier d-block elements Ru, Os, Hs are confirmed to form stable tetraoxides of Td symmetry in 1A1 electronic states with empty metal d0 valence shell and closed-shell O2− ligands, while the 3d-, 4f-, and 5f-elements Fe, Sm, and Pu prefer partial occupation of their valence shells and peroxide or superoxide ligands at lower symmetry structures with various spin couplings. The different geometric and electronic structures and chemical bonding types of the six iso-stoichiometric species are explained in terms of atomic orbital energies and orbital radii. The variations found here contribute to our general understanding of the periodic trends of oxidation states across the periodic table. type.23,24 Short-lived superheavy 6d-element Hs (269Hs(τ1/2) ∼ 1/2 min) has been theoretically shown to behave similarly to its lighter homologues.25−32 Over the years, the ground state of the lightest member 3d-FeO4 has arguably been assigned to various low-spin species, such as Td-[FeVIIIO4] (Fe-tetroxide) or C 2v /C s -[Fe VI O 2 ](O 2 ) (dioxido-Fe-peroxide or -superoxide),33−35 but various chemically viable high-spin di(su-) peroxide species of FeII,III,VI(O2)(O2) type have sometimes also been favored.5,7,36−38 A recent ab initio calculation using advanced electron correlation methods supports C2v-[FeVI (O2−)2](O2)2− as the most stable structure of FeO4, which is slightly below five other metastable isomers in energy.39 Among the 4f lanthanides, samarium with 8 valence electrons exists in trivalent form in many compounds, and divalent Sm is also known, while higher valences have not been found yet.40−42 It thus becomes a conundrum whether or not Sm can lose all its eight valence electrons as do many other elements with 8 valence electrons (including Xe),43 and whether some SmO4 species resemble some of the FeO4 species.39,44 For the 5f-actinides, plutonium has 8 valence electrons, and PuO4 has been heavily discussed during the past decades.9,45−49 Plutonium typically appears as tri-, tetra-, penta-, and hexavalent, and hepta-valence may also occur.50−56 We and others have recently found theoretical evidence that PuV in a high-spin state of C2v-[PuO2](O2) is significantly more stable than PuVIII in metastable PuO4 with average D4h symmetry.45,57−59
1. INTRODUCTION Oxidation state of elements is a central concept in chemistry.1,2 The highest identified oxidation state for the known elements (Z = 1−118) in the periodic table is VIII for neutral MO4 molecules and IX for the IrO4+ cation.3 Because of their large electronegativities, oxygen and fluorine often render the highest oxidation state of metal elements. Oxygen atoms frequently appear as divalent, doubly charged, closed-shell oxide(II) units, O2−.4 The maximum valence and oxidation state of metal atoms M increase with group number g in the periodic system up to peak values characteristic for each nth period.5−11 For instance, all group-2 and group-4 elements form stable divalent and tetravalent compounds of composition MIIO and MIVO2, respectively, and most metal atoms with 6 valence electrons form MO3, i.e., M = Cr, Mo, W, Sg, and U (except Nd).12 Concerning group-8, while RuO4 and OsO4 are well-known octavalent molecular compounds, the existence and structure of FeO4 and PuO4 are still under discussion, and SmO4 has not even been investigated.13−15 To understand why the MO4 molecules (M = Fe, Ru, Os, Hs, Sm, and Pu) of group-8 are so different, we here use quantum chemical methods to investigate (i) the vertical variation of highest metallic oxidation state within group-8 and (ii) the preference of various oxidation states of oxygen in the different MO4 species. Experimental observations have confirmed that the electronic ground states of 4d-RuO4 and 5d-OsO4 are totally symmetric spin-singlets of molecular T d symmetry with four O 2− ligands.16−24 Both compounds are amply used as markers and oxidants in organic, biological, and pharmaceutical chemistries, partly in the form of ligated complexes L2MO4 of octrahedral © 2016 American Chemical Society
Received: February 22, 2016 Published: April 13, 2016 4616
DOI: 10.1021/acs.inorgchem.6b00442 Inorg. Chem. 2016, 55, 4616−4625
Article
Inorganic Chemistry Table 1. Relativistic Atomic Effective Core Potentials (RECP) and Basis Setsa element 16
frozen full core shells
56
26Fe 102 44Ru 152 62Sm 192 76Os 244 94Pu 265 108Hs
a
Metallic element
optimized full semicore shells
2e: 1s2 10e: 1s2...2p6 28e: 1s2...3d10 28e: 1s2...3d10 60e: 1s2...4f14 60e: 1s2...4f14 78e: 1s2...5d10
8O
A ZM
optimized open valence shells 6e: 8e: 8e: 8e: 8e: 8e: 8e:
8e: (3sp)8 8e: (4sp)8 26e: (4spd)18(5sp)8 8e: (5sp)8 26e: (5spd)18(6sp)8 22e: 5f14(6sp)8
2s2p 3d4s 4d5s 4f5d6s 5d6s 5f6d7s 6d7s
RECP
basis sets
LANL68−70 LANL68−70 28MWB71 LANL68−70 60MWB74−76 CRENBL77
aug-cc-VDZ2d67 LANL2-VDZ68−70 LANL2-VDZ68−70 ANO-VTZ3g72,73 LANL2-VDZ68−70 ANO-VTZ3g75,76 CRENBL-VTZ77
with A = nuclear mass number, Z = nuclear charge number; ne = number of electrons n.
Table 2. Diagnostic Data on MO4 Electronic Ground State Wave Functions CCSD(T)b molecule SmIII(O2)(O2•) [PuVO2](O2•) [FeVIO2](O2) RuVIIIO4 OsVIIIO4 HsVIIIO4
B3LYP
ΔεHLa
2.76 1.76 2.79 3.27 4.12 4.70
CASPT2c
T1
D1
AS
C0
C1
0.05 0.04 0.05 0.03 0.03 0.06
0.17 0.15 0.20 0.10 0.08 0.17
(12e, 12o)d (12e, 12o)e (24e, 17o)f (12e, 12o)g (12e, 12o)g (12e, 12o)g
86% 89% 64% 89% 93% 81%
1% 1% 4% 1%
