title: Theoretical Studies on Hexanuclear Oxometalates [M6L19](q-) (M = Cr, Mo, W, Sg, Nd, U). Electronic Structures, Oxidation States, Aromaticity, and Stability.
abstract: We here report a systematic theoretical study on geometries, electronic structures, and energetic stabilities of six hexanuclear polyoxometalates [M6O19](2-) of the six-valence-electron metals including the d-elements M = Cr, Mo, W, Sg from group 6 and the f-elements M = Nd, U. Scalar relativistic density functional theory was applied to these clusters in vacuum and in solution. It is shown that the Oh Lindqvist structure of the isolated [M6O19](2-) units with hexavalent M elements (M(+6)) is only stable for the three heavy transition metals M = Mo, W, and Sg. The rare Th symmetry is predicted for M = U both in vacuum and in solution, owing to pseudo-Jahn-Teller distortion of these closed-shell systems. The Oh and Th structures correspond to cyclic "aromatic" U-̇O-̇U and alternating U=O-U bonding of cross-linked U4O4 rings, respectively. The reduced [U6O19](8-) cluster with pentavalent U(+5) also shows Th symmetry in vacuum, but Oh symmetry in a dielectric environment. The occurrence of different structures for varying fractional oxidation states in different environments is rationalized. Theoretical investigation of the recently synthesized U(+5) complex [U6O13L6](0) (L6 = tetracyclopentadienyl dibipyridine) shows a distorted Th-type symmetry, too. The stabilities of these complexes of different metal oxidation states are consistent with the general periodic trends of oxidation states.
authors.affiliation: §Department of Chemistry and Key Laboratory of Organic Optoelectronics and Molecular Engineering, Ministry of Education, Tsinghua University, Beijing 100084, China.
authors.forename: Ning
authors.initials: N
authors.lastname: Jiang
authors.affiliation: §Department of Chemistry and Key Laboratory of Organic Optoelectronics and Molecular Engineering, Ministry of Education, Tsinghua University, Beijing 100084, China.|∥Physical and Theoretical Chemistry, University of Siegen, Siegen 57068, Germany.
authors.forename: W H Eugen
authors.initials: WH
authors.lastname: Schwarz
authors.affiliation: §Department of Chemistry and Key Laboratory of Organic Optoelectronics and Molecular Engineering, Ministry of Education, Tsinghua University, Beijing 100084, China.
authors.forename: Jun
authors.initials: J
authors.lastname: Li