title: From Widely Accepted Concepts in Coordination Chemistry to Inverted Ligand Fields.
abstract: We begin with a brief historical review of the development of our understanding of the normal ordering of nd orbitals of a transition metal interacting with ligands, the most common cases being three below two in an octahedral environment, two below three in tetrahedral coordination, and four below one in a square-planar environment. From the molecular orbital construction of these ligand field splittings evolves a strategy for inverting the normal order: the obvious way to achieve this is to raise the ligand levels above the metal d's; that is, make the ligands better Lewis bases. However, things are not so simple, for such metal/ligand level placement may lead to redox processes. For 18-electron octahedral complexes one can create the inverted situation, but it manifests itself in the makeup of valence orbitals (are they mainly on metal or ligands?) rather than energy. One can also see the effect, in small ways, in tetrahedral Zn(II) complexes. We construct several examples of inverted ligand field systems with a hypothetical but not unrealistic AlCH3 ligand and sketch the consequences of inversion on reactivity. Special attention is paid to the square-planar case, exemplified by [Cu(CF3)4](-), in which Snyder had the foresight to see a case of an inverted field, with the empty valence orbital being primarily ligand centered, the dx2-y2 orbital heavily occupied, in what would normally be called a Cu(III) complex. For [Cu(CF3)4](-) we provide theoretical evidence from electron distributions, geometry of the ligands, thermochemistry of molecule formation, and the energetics of abstraction of a CF3 ligand by a base, all consistent with oxidation of the ligands in this molecule. In [Cu(CF3)4](-), and perhaps more complexes on the right side of the transition series than one has imagined, some ligands are σ-noninnocent. Exploration of inverted ligand fields helps us see the continuous, borderless transition from transition metal to main group bonding. We also give voice to a friendly disagreement on oxidation states in these remarkable molecules.
authors.affiliation: Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University , Ithaca, New York 14853, United States.
authors.forename: Roald
authors.initials: R
authors.lastname: Hoffmann
authors.affiliation: Departament de Química Inorgànica and Institut de Química Teòrica i Computacional, Universitat de Barcelona , Martí i Franquès 1, 08028 Barcelona, Spain.
authors.forename: Santiago
authors.initials: S
authors.lastname: Alvarez
authors.affiliation: Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti Organometallici (CNR-ICCOM) , Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy.
authors.forename: Carlo
authors.initials: C
authors.lastname: Mealli
authors.affiliation: Departament de Química Inorgànica and Institut de Química Teòrica i Computacional, Universitat de Barcelona , Martí i Franquès 1, 08028 Barcelona, Spain.
authors.forename: Andrés
authors.initials: A
authors.lastname: Falceto
authors.affiliation: Department of Biochemistry, Duke University , Durham, North Carolina 27710, United States.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Cahill
authors.affiliation: Department of Chemistry, Carleton University , Ottawa, ON K1S5B6, Canada.
authors.forename: Tao
authors.initials: T
authors.lastname: Zeng
authors.affiliation: Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti Organometallici (CNR-ICCOM) , Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy.
authors.forename: Gabriele
authors.initials: G
authors.lastname: Manca
mesh_terms: D040761:Biomimetic Materials / Q000737:chemistry; D056831:Coordination Complexes / Q000737:chemistry*; D058115:Lewis Bases / Q000737:chemistry; D008024:Ligands; D019216:Metals, Heavy / Q000737:chemistry; D019565:Metals, Light / Q000737:chemistry; D008956:Models, Chemical; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011789:Quantum Theory
chemical_list: D056831:Coordination Complexes; D058115:Lewis Bases; D008024:Ligands; D019216:Metals, Heavy; D019565:Metals, Light