On the Nature of the Bonding in 1:1 Adducts of O₂

A survey of the potential energy surface for a 1:1 copper dioxygen complex, (C3N2H5)CuO2, reveals two distinct states in the valence region, a singlet (1A1) and a triplet (3B1). The former spans a continuum from CuIII−O22- to CuI−O2(1Δg), while the latter spans CuII−O21- to CuI−O2(3Σg-). The point at which the potential energy curves for the two states cross marks an abrupt discontinuity in electron distribution, where the system shifts from dominant CuIII−O22- character to CuII−O21-. On this basis, we argue that there is no continuum between CuIII−peroxide and CuII−superoxide:  the two are represented by distinct states that differ both in symmetry and multiplicity

On the Electronic Origins of Structural Isomerism in the Iron−Sulfur Cubane, [(C₅H₅)₄Fe₄S₄]²⁺

Density functional theory provides new insights into the structural isomerism observed in the cyclopentadienyl-capped iron−sulfur cluster, [(C5H5)4Fe4S4]2+. Two distinct, closely spaced minima have been located, a triplet with D2 symmetry and a C2-symmetric singlet, both of which correspond closely to the structure of one of the known crystal forms of the cation. Thus, the structural diversity in these species reflects genuine molecular bistability rather than simple solid-state packing effects. In contrast, no stable D2d-symmetric minimum has been located, suggesting that the reported D2d symmetry of the cation in [(C5H5)4Fe4S4][PF6]2 may be a crystallographic artifact. In the ruthenium analogue, the more diffuse 4d orbitals stabilize the C2-symmetric singlet, which is unambiguously the ground state, but the D2-symmetric potential energy surface provides a viable low-energy pathway for the dynamic exchange of the Ru−Ru bonds

Efficient Spin Filtering through Cobalt-Based Extended Metal Atom Chains

Density functional theory in conjunction with nonequilibrium Green’s functions has been used to explore charge transport through the cobalt-based extended metal atom chain, Co3(dpa)4(NCS)2. The isolated molecule has a doublet ground state, and the singly occupied σ nonbonding orbital proves to be the dominant transport channel, providing spin filtering efficiencies in excess of 90%. The metal chain differs from typical organic conductors in that the π orbitals that form the contact with the gold electrode are orthogonal to the transport channel. As a result, the rehybridization of these π levels by the applied electric field has only a minor impact on the current, allowing spin filtering to persist even at biases in excess of 1 V

Influence of Low-Symmetry Distortions on Electron Transport through Metal Atom Chains: When Is a Molecular Wire Really “Broken”?

In the field of molecular electronics, an intimate link between the delocalization of molecular orbitals and their ability to support current flow is often assumed. Delocalization, in turn, is generally regarded as being synonymous with structural symmetry, for example, in the lengths of the bonds along a molecular wire. In this work, we use density functional theory in combination with nonequilibrium Green’s functions to show that precisely the opposite is true in the extended metal atom chain Cr3(dpa)4(NCS)2 where the delocalized π framework has previously been proposed to be the dominant conduction pathway. Low-symmetry distortions of the Cr3 core do indeed reduce the effectiveness of these π channels, but this is largely irrelevant to electron transport at low bias simply because they lie far below the Fermi level. Instead, the dominant pathway is through higher-lying orbitals of σ symmetry, which remain essentially unperturbed by even quite substantial distortions. In fact, the conductance is actually increased marginally because the σnb channel is displaced upward toward the Fermi level. These calculations indicate a subtle and counterintuitive relationship between structure and function in these metal chains that has important implications for the interpretation of data emerging from scanning tunnelling and atomic force microscopy experiments

A Three-State Model for the Polymorphism in Linear Tricobalt Compounds

The remarkable polymorphism exhibited by the linear tricobalt compounds, Co3(μ3-dpa)4Cl2 and Co3(μ3-dpa)4Br2, can be explained using a model involving three distinct electronic states. At high temperatures, symmetric and unsymmetric forms arise from the population of doublet (2A) and quartet (4B) states, respectively, the latter containing a localized high-spin CoII center. In the unsymmetric form, a reduction in temperature leads to a spin-crossover to a second quite distinct doublet state, 2B, where, uniquely, the dx2-y2 character on the localized CoII center is distributed between the occupied and vacant manifolds. The variable population of the Co dx2-y2 orbital gives rise to the continuous change in Co−Co and Co−N bond lengths as the temperature is decreased

On the Nature of the Bonding in 1:1 Adducts of O₂

A survey of the potential energy surface for a 1:1 copper dioxygen complex, (C3N2H5)CuO2, reveals two distinct states in the valence region, a singlet (1A1) and a triplet (3B1). The former spans a continuum from CuIII−O22- to CuI−O2(1Δg), while the latter spans CuII−O21- to CuI−O2(3Σg-). The point at which the potential energy curves for the two states cross marks an abrupt discontinuity in electron distribution, where the system shifts from dominant CuIII−O22- character to CuII−O21-. On this basis, we argue that there is no continuum between CuIII−peroxide and CuII−superoxide:  the two are represented by distinct states that differ both in symmetry and multiplicity

On the Electronic Origins of Structural Isomerism in the Iron−Sulfur Cubane, [(C₅H₅)₄Fe₄S₄]²⁺

Density functional theory provides new insights into the structural isomerism observed in the cyclopentadienyl-capped iron−sulfur cluster, [(C5H5)4Fe4S4]2+. Two distinct, closely spaced minima have been located, a triplet with D2 symmetry and a C2-symmetric singlet, both of which correspond closely to the structure of one of the known crystal forms of the cation. Thus, the structural diversity in these species reflects genuine molecular bistability rather than simple solid-state packing effects. In contrast, no stable D2d-symmetric minimum has been located, suggesting that the reported D2d symmetry of the cation in [(C5H5)4Fe4S4][PF6]2 may be a crystallographic artifact. In the ruthenium analogue, the more diffuse 4d orbitals stabilize the C2-symmetric singlet, which is unambiguously the ground state, but the D2-symmetric potential energy surface provides a viable low-energy pathway for the dynamic exchange of the Ru−Ru bonds

On the Electronic Origins of Structural Isomerism in the Iron−Sulfur Cubane, [(C₅H₅)₄Fe₄S₄]²⁺

Density functional theory provides new insights into the structural isomerism observed in the cyclopentadienyl-capped iron−sulfur cluster, [(C5H5)4Fe4S4]2+. Two distinct, closely spaced minima have been located, a triplet with D2 symmetry and a C2-symmetric singlet, both of which correspond closely to the structure of one of the known crystal forms of the cation. Thus, the structural diversity in these species reflects genuine molecular bistability rather than simple solid-state packing effects. In contrast, no stable D2d-symmetric minimum has been located, suggesting that the reported D2d symmetry of the cation in [(C5H5)4Fe4S4][PF6]2 may be a crystallographic artifact. In the ruthenium analogue, the more diffuse 4d orbitals stabilize the C2-symmetric singlet, which is unambiguously the ground state, but the D2-symmetric potential energy surface provides a viable low-energy pathway for the dynamic exchange of the Ru−Ru bonds

First Structural Characterization of a Delocalized, Mixed-Valent, Triangular Cu₃⁷⁺ Species: Chemical and Electrochemical Oxidation of a Cu^II₃(μ₃-O) Pyrazolate and Electronic Structure of the Oxidation Product

The chemical or electrochemical one-electron oxidation of the all-CuII complex [Cu3(μ3-O)(μ-pz)3X3]2- leads to its formally CuII2CuIII analogue (pz = pyrazolato anion; X = Cl- and PhCOO-). The X-ray single-crystal structure and density functional theory analysis of the latter agree in revealing the delocalized nature of its mixed-valent Cu37+ core

First Structural Characterization of a Delocalized, Mixed-Valent, Triangular Cu₃⁷⁺ Species: Chemical and Electrochemical Oxidation of a Cu^II₃(μ₃-O) Pyrazolate and Electronic Structure of the Oxidation Product

The chemical or electrochemical one-electron oxidation of the all-CuII complex [Cu3(μ3-O)(μ-pz)3X3]2- leads to its formally CuII2CuIII analogue (pz = pyrazolato anion; X = Cl- and PhCOO-). The X-ray single-crystal structure and density functional theory analysis of the latter agree in revealing the delocalized nature of its mixed-valent Cu37+ core