title: Orbital entanglement and CASSCF analysis of the Ru-NO bond in a Ruthenium nitrosyl complex.
abstract: Complete active space self-consistent field (CASSCF) wavefunctions and an orbital entanglement analysis obtained from a density-matrix renormalisation group (DMRG) calculation are used to understand the electronic structure, and, in particular, the Ru-NO bond of a Ru nitrosyl complex. Based on the configurations and orbital occupation numbers obtained for the CASSCF wavefunction and on the orbital entropy measurements evaluated for the DMRG wavefunction, we unravel electron correlation effects in the Ru coordination sphere of the complex. It is shown that Ru-NO π bonds show static and dynamic correlation, while other Ru-ligand bonds feature predominantly dynamic correlation. The presence of static correlation requires the use of multiconfigurational methods to describe the Ru-NO bond. Subsequently, the CASSCF wavefunction is analysed in terms of configuration state functions based on localised orbitals. The analysis of the wavefunctions in the electronic singlet ground state and the first triplet state provides a picture of the Ru-NO moiety beyond the standard representation based on formal oxidation states. A distinct description of the Ru and NO fragments is advocated. The electron configuration of Ru is an equally weighted superposition of Ru(II) and Ru(III) configurations, with the Ru(III) configuration originating from charge donation mostly from Cl ligands. However, and contrary to what is typically assumed, the electronic configuration of the NO ligand is best described as electroneutral.
authors.affiliation: Institut für theoretische Chemie, Universität Wien, Währinger Str. 17, 1090 Vienna, Austria. leticia.gonzalez@univie.ac.at.
authors.forename: Leon
authors.initials: L
authors.lastname: Freitag
authors.forename: Stefan
authors.initials: S
authors.lastname: Knecht
authors.forename: Sebastian F
authors.initials: SF
authors.lastname: Keller
authors.forename: Mickaël G
authors.initials: MG
authors.lastname: Delcey
authors.forename: Francesco
authors.initials: F
authors.lastname: Aquilante
authors.forename: Thomas Bondo
authors.initials: TB
authors.lastname: Pedersen
authors.forename: Roland
authors.initials: R
authors.lastname: Lindh
authors.forename: Markus
authors.initials: M
authors.lastname: Reiher
authors.forename: Leticia
authors.initials: L
authors.lastname: González
mesh_terms: D000465:Algorithms; D001665:Binding Sites; D002412:Cations; D003198:Computer Simulation; D007191:Indazoles / Q000737:chemistry*; D008956:Models, Chemical; D062105:Molecular Docking Simulation; D009569:Nitric Oxide / Q000737:chemistry*; D009716:Numerical Analysis, Computer-Assisted; D009942:Organometallic Compounds / Q000737:chemistry*; D011789:Quantum Theory; D012428:Ruthenium / Q000737:chemistry*; D017975:Ruthenium Compounds
chemical_list: D002412:Cations; D007191:Indazoles; D009942:Organometallic Compounds; D017975:Ruthenium Compounds; C499292:indazolium trans-(tetrachlorobis(1H-indazole)ruthenate (III)); D009569:Nitric Oxide; D012428:Ruthenium