An Objective Alternative to IUPAC's Approach To Assign Oxidation States

Unraveling the Mechanism of Hydrogen Atom Transfer by a Nickel-Hypochlorite Species and the Influence of Electronic Effects

The oxidation of hydrocarbons is an important chemical transformation with relevance to biology and industry. Ni-catalyzed transformations are more scarce compared to Mn or Fe but have gained attention in recent years, affording efficient oxidations. Understanding the mechanism of action of these catalysts, including the detection and characterization of the active nickel-oxygen species, is of interest to design better catalysts. In this work, we undertake a theoretical study to unravel the mechanism of formation of the previously reported [Ni(OCl)(HL)]+ (H2) and how it activates C-H bonds. We disclose that the active species is indeed compound [Ni(O)(HL)]+, formed after homolytic cleavage of the O-Cl bond in H2 assisted by a chlorine radical. [Ni(O)(HL)]+ mediates C-H activation through an asynchronous concerted mechanism, in which the transition state is given by hydrogen atom transfer. Moreover, the electronic tuning of the ligand has a very modest impact on the stability and reactivity of the corresponding X2 species. Effective oxidation state analysis reveals an intriguing electronic structure of H2 and [Ni(O)(HL)]+, in which both the macrocycic HL ligand and the OCl and O ligands behave as redox noninnocent. Such redox activity leads to a fully ambiguous oxidation state assignation.

Oxidation States: Intrinsically Ambiguous?

The oxidation state ( OS ) formalism is a much-appreciated good in chemistry, receiving wide application. However, like all formalisms, limitations are inescapable, some of which have been recently explored. Providing a broader context, we discuss the OS and its interpretation from a computational perspective for transition metal (TM) complexes. We define a broadly applicable and easy-to-use procedure to derive OS s based on quantum chemical calculations, via the use of localized orbitals, dubbed the Intrinsic OS . Applying this approach to a cobalt complex in five OS s, isolated by Hunter and co-workers (Inorg. Chem.2021, 60, 17445), we find that the calculated Intrinsic OS matches the formal OS , consistent with the experimental characterization. Through analysis of the delocalized orbitals, the ligand field of the Co(III) complex is found to be "inverted", despite every cobalt-ligand bond being classically dative from the localized perspective-a bonding scenario very similar to that of [Cu(CF3)4]-. This is not atypical but rather a natural consequence of these metals bonding in the high-valent region, and we propose a more restrictive definition of (locally) inverted bonding. Additionally, two bonding descriptors within the Intrinsic Bonding Orbital (IBO) framework (σ-gain and π-loss) are introduced, which enable facile quantification of electron-sharing covalency across a broad range of TM complexes.

Similarities and Differences in Benzene Reduction with Ca, Sr, Yb and Sm: Strong Evidence for Tetra-Anionic Benzene

Inverse sandwich complexes of Yb and Sm stabilized by a bulky β-diketiminate (BDI) ligand have been prepared: (BDI)Ln(η6,η6-C6H6)Ln(BDI); Ln=lanthanide. Coordinated benzene ligands can be neutral, di-anionic or, often controversially discussed, even tetra-anionic. The formal charge on benzene is correlated to assignment of the metal oxidation state which generally poses a problem. Herein, we take advantage of the structural similarities found when comparing CaII with YbII, and SrII with SmII complexes. In this work, we found an excellent overlap of the Ca/Yb inverse sandwich structures but a striking difference for the Sr/Sm pair. The much shorter Sm-N and Sm-C6H6 distances are strong evidence for a SmIII-benzene-4-SmIII assignment. This was further supported by NMR spectroscopy, magnetic susceptibility, reactivity and comprehensive computational investigation.

Quantification of the Donor-Acceptor Character of Ligands by the Effective Fragment Orbitals

Metal-ligand interactions are at the heart of transition metal complexes. The Dewar-Chat-Duncanson model is often invoked, whereby the metal-ligand bonding is decomposed into the simultaneous ligand→metal electron donation and the metal→ligand back-donation. The separate quantification of both effects is not a trivial task, neither from experimental nor computational exercises. In this work we present the effective fragment orbitals (EFOs) and their occupations as a general procedure beyond the Kohn-Sham density functional theory (KS-DFT) framework for the identification and quantification of donor-acceptor interactions, using solely the wavefunction of the complex. Using a common Fe(II) octahedral complex framework, we quantify the σ-donor, π-donor, and π-acceptor character for a large and chemically diverse set of ligands, by introducing the respective descriptors σd, πd, and πa. We also explore the effect of the metal size, coordination number, and spin state on the donor/acceptor features. The spin-state is shown to be the most critical effect, inducing a systematic decrease of the sigma donation and π-backdonation going from low spin to high spin. Finally, we illustrate the ability of the EFOs to rationalize the Tolman electronic parameter and the trans influence in planar square complexes in terms of these new descriptors.

How electrons still guard the space: Electron number distribution functions based on QTAIM∩ELF intersections

Despite the importance of the one-particle picture provided by the orbital paradigm, a rigorous understanding of the spatial distribution of electrons in molecules is still of paramount importance to chemistry. Considerable progress has been made following the introduction of topological approaches, capable of partitioning space into chemically meaningful regions. They usually provide atomic partitions, for example, through the attraction basins of the electron density in the quantum theory of atoms in molecules (QTAIM) or electron-pair decompositions, as in the case of the electron localization function (ELF). In both cases, the so-called electron distribution functions (EDFs) provide a rich statistical description of the electron distribution in these spatial domains. Here, we take the EDF concept to a new fine-grained limit by calculating EDFs in the QTAIM ∩ ELF intersection domains. As shown in AHn systems based on main group elements, as well as in the CO, NO, and BeO molecules, this approach provides an exquisitely detailed picture of the electron distribution in molecules, allowing for an insightful combination of the distribution of electrons between Lewis entities (such as bonds and lone pairs) and atoms at the same time. Besides mean-field calculations, we also explore the impact of electron correlation through Hartree-Fock (HF), density functional theory (DFT) (B3LYP), and CASSCF calculations.

T-Shaped Palladium and Platinum {MNO}10 Nitrosyl Complexes

The synthesis and characterization of a homologous series of T-shaped {MNO}10 nitrosyl complexes of the form [M(PR3)2(NO)]+ (M = Pd, Pt; R = tBu, Ad) are reported. These diamagnetic nitrosyls are obtained from monovalent or zerovalent precursors by treatment with NO and NO+, respectively, and are notable for distinctly bent M-NO angles of ∼123° in the solid state. Adoption of this coordination mode in solution is also supported by the analysis of isotopically enriched samples by 15N NMR spectroscopy. Effective oxidation states of M0/NO+ are calculated, and metal-nitrosyl bonding has been interrogated using DFT-based energy decomposition analysis techniques. While a linear nitrosyl coordination mode was found to be electronically preferred, the M-NO and P-M-P angles are inversely correlated to the extent that binding in this manner is prevented by steric repulsion between the bulky ancillary phosphine ligands.

On the highest oxidation states of the actinoids in AnO4 molecules (An = Ac - Cm): A DMRG-CASSCF study

Actinoid tetroxide molecules AnO₄ (An = Ac - Cm) are investigated with the ab initio density matrix renormalization group (DMRG) approach. Natural orbital shapes are used to read out the oxidation state (OS) of the f-elements, and the atomic orbital energies and radii are used to explain the trends. The highest OSs reveal a "volcano"-type variation: For An = Ac - Np, the OSs are equal to the number of available valence electrons, that is, Ac^III , Th^IV , Pa^V , U^VI , and Np^VII . Starting with plutonium as the turning point, the highest OSs in the most stable AnO₄ isomers then decrease as Pu^V , Am^V , and Cm^III , indicating that the 5f-electrons are hard to be fully oxidized off from Pu onward. The variations are related to the actinoid contraction and to the 5f-covalency characteristics. Combined with previous work on OSs, we review their general trends throughout the periodic table, providing fundamental understanding of OS-relevant phenomena.

Characterization of a Ferryl Flip in Electronically Tuned Nonheme Complexes. Consequences in Hydrogen Atom Transfer Reactivity

Two oxoiron(IV) isomers (^R 2a and ^R 2b) of general formula [Fe^IV (O)(^R PyNMe₃ )(CH₃ CN)]²⁺ are obtained by reaction of their iron(II) precursor with NBu₄ IO₄ . The two isomers differ in the position of the oxo ligand, cis and trans to the pyridine donor. The mechanism of isomerization between ^R 2a and ^R 2b has been determined by kinetic and computational analyses uncovering an unprecedented path for interconversion of geometrical oxoiron(IV) isomers. The activity of the two oxoiron(IV) isomers in hydrogen atom transfer (HAT) reactions shows that ^R 2a reacts one order of magnitude faster than ^R 2b, which is explained by a repulsive noncovalent interaction between the ligand and the substrate in ^R 2b. Interestingly, the electronic properties of the R substituent in the ligand pyridine ring do not have a significant effect on reaction rates. Overall, the intrinsic structural aspects of each isomer define their relative HAT reactivity, overcoming changes in electronic properties of the ligand.

Reply to the 'Comment on "The oxidation state in low-valent beryllium and magnesium compounds"' by S. Pan and G. Frenking, Chem. Sci., 2022, , DOI: 10.1039/D2SC04231B

A recent article by Pan and Frenking challenges our assignment of the oxidation state of low valent group 2 compounds. With this reply, we show that our assignment of Be(+2) and Mg(+2) oxidation states in Be(cAAC^Dip)₂ and Mg(cAAC^Dip)₂ is fully consistent with our data. Some of the arguments exposed by Pan and Frenking were based on visual inspection of our figures, rather than a thorough numerical analysis. We discuss with numerical proof that some of the statements made by the authors concerning our reported data are erroneous. In addition, we provide further evidence that the criterion of the lowest orbital interaction energy in the energy decomposition analysis (EDA) method is unsuitable as a general tool to assess the valence state of the fragments. Other indicators based on natural orbitals for chemical valence (NOCV) deliver a more reliable bonding picture. We also emphasize the importance of using stable wavefunctions for any kind of analysis, including EDA.

How reduced are nucleophilic gold complexes?

Nucleophilic formal gold(-I) and gold(I) complexes are investigated via Intrinsic Bond Orbital analysis and Energy Decomposition Analysis, based on density functional theory calculations. The results indicate gold(0) centres engaging in electron-sharing bonding with Al- and B- based ligands. Multiconfigurational (CASSCF) calculations corroborate the findings, highlighting the gap between the electonic structures and the oxidation state formalism.

Pas de Deux of an NO Couple: Synchronous Photoswitching from a Double-Linear to a Double-Bent Ru(NO)2 Core under Nitrosyl Charge Conservation

The {Ru(NO)₂ }¹⁰ dinitrosylruthenium complex [Ru(NO)₂ (PPh₃ )₂ ] (1) shows photo-induced linkage isomerism (PLI) of a special kind: the two NO ligands switch, on photo-excitation, synchronously from the ground state (GS) with two almost linear RuNO functions to a metastable state (MS) which persists up to 230 K and can be populated to ≈50 %. The MS was experimentally characterised by photo-crystallography, IR spectroscopy and DS-calorimetry as a double-bent variant of the double-linear GS. The experimental results are confirmed by computation which unravels the GS/MS transition as a disrotatory synchronous 50° turn of the two nitrosyl ligands. Although 1 shows the usual redshift of the N-O stretch on bending the MNO unit, there is no increased charge transfer from Ru to NO along the GS-to-MS path. In terms of the effective-oxidation-state (EOS) method, both isomers of 1 and the transition state are Ru^-II (NO⁺ )₂ species.

Carboxylic Acid Directed γ-Lactonization of Unactivated Primary C-H Bonds Catalyzed by Mn Complexes: Application to Stereoselective Natural Product Diversification

Reactions that enable selective functionalization of strong aliphatic C-H bonds open new synthetic paths to rapidly increase molecular complexity and expand chemical space. Particularly valuable are reactions where site-selectivity can be directed toward a specific C-H bond by catalyst control. Herein we describe the catalytic site- and stereoselective γ-lactonization of unactivated primary C-H bonds in carboxylic acid substrates. The system relies on a chiral Mn catalyst that activates aqueous hydrogen peroxide to promote intramolecular lactonization under mild conditions, via carboxylate binding to the metal center. The system exhibits high site-selectivity and enables the oxidation of unactivated primary γ-C-H bonds even in the presence of intrinsically weaker and a priori more reactive secondary and tertiary ones at α- and β-carbons. With substrates bearing nonequivalent γ-C-H bonds, the factors governing site-selectivity have been uncovered. Most remarkably, by manipulating the absolute chirality of the catalyst, γ-lactonization at methyl groups in gem-dimethyl structural units of rigid cyclic and bicyclic carboxylic acids can be achieved with unprecedented levels of diastereoselectivity. Such control has been successfully exploited in the late-stage lactonization of natural products such as camphoric, camphanic, ketopinic, and isoketopinic acids. DFT analysis points toward a rebound type mechanism initiated by intramolecular 1,7-HAT from a primary γ-C-H bond of the bound substrate to a highly reactive Mn^IV-oxyl intermediate, to deliver a carbon radical that rapidly lactonizes through carboxylate transfer. Intramolecular kinetic deuterium isotope effect and ¹⁸O labeling experiments provide strong support to this mechanistic picture.

Ni(I)-TPA Stabilization by Hydrogen Bond formation on the Second Coordination Sphere: a DFT Characterization

Ni(I) compounds are less common than those of either Ni(0) or Ni(II). Recently, a series of Ni(I) tris(2 pyridylmethyl)amine (TPA) complexes were synthetized through the reduction of Ni(II)-TPA complexes and...

Impact of the coordination of multiple Lewis acid functions on the electronic structure and vn configuration of a metal center

The covalent bond classification (CBC) method represents a molecule as ML_lX_xZ_z by evaluating the total number of L, X and Z functions interacting with M. The CBC method is a simplistic approach that is based on the notion that the bonding of a ligating atom (or group of atoms) can be expressed in terms of the number of electrons it contributes to a 2-electron bond. In many cases, the bonding in a molecule of interest can be described in terms of a 2-center 2-electron bonding model and the ML_lX_xZ_z classification can be derived straightforwardly by considering each ligand independently. However, the bonding within a molecule cannot always be described satisfactorily by using a 2-center 2-electron model and, in such situations, the ML_lX_xZ_z classification requires a more detailed consideration than one in which each ligand is treated in an independent manner. The purpose of this article is to provide examples of how the ML_lX_xZ_z classification is obtained in the presence of multicenter bonding interactions. Specific emphasis is given to the treatment of multiple π-acceptor ligands and the impact on the vⁿ configuration, i.e. the number of formally nonbonding electrons on an element of interest.

Oxidation State Localized Orbitals: A Method for Assigning Oxidation States Using Optimally Fragment-Localized Orbitals and a Fragment Orbital Localization Index

Oxidation states represent the ionic distribution of charge in a molecule and are significant in tracking redox reactions and understanding chemical bonding. While effective algorithms already exist based on formal Lewis structures as well as using localized orbitals, they exhibit differences in challenging cases where effects such as redox noninnocence are at play. Given a density functional theory (DFT) calculation with chosen total charge and spin multiplicity, this work reports a new approach to obtaining fragment-localized orbitals that is termed oxidation state localized orbitals (OSLO), together with an algorithm for assigning the oxidation state using the OSLOs and an associated fragment orbital localization index (FOLI). Evaluating the FOLI requires fragment populations, and for this purpose a new version of the intrinsic atomic orbital (IAO) scheme is introduced in which the IAOs are evaluated using a reference minimal basis formed from on-the-fly superposition of atomic density (IAO-AutoSAD) calculations in the target basis set and at the target level of theory. The OSLO algorithm is applied to a range of challenging cases including high valent metal oxide complexes, redox noninnocent NO and dithiolate transition metal complexes, a range of carbene-containing TM complexes, and other examples including the potentially inverted ligand field in [Cu(CF₃)₄]^-. Across this range of cases, OSLO produces generally satisfactory results. Furthermore, in borderline cases, the OSLOs and associated FOLI values provide direct evidence of the emergence of covalent interactions between fragments that nicely complements existing approaches.

Unveiling the Electronic Structure of the Bi(+1)/Bi(+3) Redox Couple on NCN and NNN Pincer Complexes

Low-valent group 15 compounds stabilized by pincer ligands have gained particular interest, given their direct access to fine-tune their reactivity by the coordination pattern. Recently, bismuth has been employed in a variety of catalytic transformations by taking advantage of the (+1/+3) redox couple. In this work, we present a detailed quantum–chemical study on the electronic structure of bismuth pincer complexes from two different families, namely, bis(ketimine)phenyl (NCN) and triamide bismuthinidene (NNN). The use of the so-called effective oxidation state analysis allows the unambiguous assignation of the bismuth oxidation state. In contrast to previous studies, our calculations suggest a Bi(+1) assignation for NCN pincer ligands, while Bi(+3) character is found for NNN pincer complexes. Notably, regardless of its oxidation state, the central bismuth atom disposes of up to two lone pairs for coordinating Lewis acids, as indicated by very high first and second proton affinity values. Besides, the Bi–NNN systems can also accommodate two Lewis base ligands, indicating also ambiphilic behavior. The effective fragment orbital analysis of Bi and the ligand allows monitoring of the intricate electron flow of these processes, revealing the noninnocent nature of the NNN ligand, in contrast with the NCN one. By the dissection of the electron density into effective fragment orbitals, we are able to quantify and rationalize the Lewis base/acid character.

Effective oxidation state analysis sheds light on the electronic structure of chemical systems. The oxidation state of bismuthinidene pincer complexes can be assigned as Bi(+1) or Bi(+3) depending on the nature of the ligands. Despite this assignation, the reactivity pattern as Lewis base or acid is similar. The occupation of the effective fragment orbitals gives a straightforward method to quantify the reactivity.

How Aromatic Are Molecular Nanorings? The Case of a Six-Porphyrin Nanoring*

Large conjugated rings with persistent currents are novel promising structures in molecular-scale electronics. A six-porphyrin nanoring structure that allegedly sustained an aromatic ring current involving 78π electrons was recently synthesized. We provide here compelling evidence that this molecule is not aromatic, contrary to what was inferred from the analysis of 1 H-NMR data and computational calculations that suffer from large delocalization errors. The main reason behind the absence of an aromatic ring current in these nanorings is the low delocalization in the transition from the porphyrins to the bridging butadiyne linkers, which disrupts the overall conjugated circuit. These results highlight the importance of choosing a suitable computational method to study large conjugated molecules and the appropriate aromaticity descriptors to identify the part of the molecule responsible for the loss of aromaticity.

Bent and Linear {CoNO}8 Entities: Structure and Bonding in a Prototypic Class of Nitrosyls

Among the isoelectronic ligands CN^-, CO, and NO⁺, an oblique bonding to the metal is well-established for the nitrosyl ligand, with M-N-O angles down to ≈120°. In the last decades, the nitrosyl community got into the habit of addressing a bent-bonded nitrosyl ligand as ¹NO^-. Thus, because various redox forms of a nitrosyl ligand seem to exist, the ligand is considered to be "noninnocent" because of the obvious ambiguity of an oxidation state (OS) assignment of the ligand and metal. Among the bent-bonded species, the low-spin {CoNO}⁸ class is prototypic. From this class, some 20 new nitrosyl compounds, the X-ray structure determinations of which comply with strict quality criteria, were analyzed with respect to the OS issue. As a result, the effective OS method shows a low-spin d⁸ Co^I-NO⁺ couple instead of a negative OS of the ligand at the BP86/def2-TZVP (+D3, +CPCM with infinite permittivity) level of theory. The same holds for some new members of the linear subclass of {CoNO}⁸ compounds. For all compounds, a largely invariable "real" charge of ≈ -0.3 e was obtained from population analyses. All of these electron-rich d⁸ species strive to manage Pauli repulsion between the metal electrons and the lone pair at the nitrosyl's nitrogen atom, with the bending of the CoNO unit as the most frequent escape.

Electronic Structure and Spectroscopic Properties of Group-7 Tri-Oxo-Halides MO3X (M = Mn-Bh, X = F-Ts)

The 24 trioxide halide molecules MO₃X of the manganese group (M = Mn-Bh; X = F-Ts), which are iso-valence-electronic with the famous MnO_4^- ion, have been quantum-chemically investigated by quasi-relativistic density-functional and ab initio correlated approaches. Geometric and electronic structures, valence and oxidation numbers, vibrational and electronic spectral properties, energetic stabilities of the monomers in the gas phase, and the decay mode of MnO₃F have been investigated. The light Mn-3d species are most strongly electron-correlated, indicating that the concept of a closed-shell Lewis-type single-configurational structure [Mn⁺⁷(d⁰) O^-2(p⁶)₃ F^-(p⁶)] reaches its limits. The concept of real-valued spin orbitals φ(r)·α and φ(r)·β breaks down for the heavy Bh-6d, At-6p and Ts-7p elements because of the dominating spin-orbit coupling. The vigorous decomposition of MnO₃F at ambient conditions starts by the autocatalyzed release of n O₂ and the formation of Mn_mO_3m-2nF_m clusters, triggered by the electron-depleted "oxylic" character of the oxide ligands in MnO₃X.

Chasing the Mond Cation: Synthesis and Characterization of the Homoleptic Nickel Tetracarbonyl Cation and its Tricarbonyl-Nitrosyl Analogue

130 years after Mond discovered the first homoleptic carbonyl complex Ni(CO)₄, we report on a [Ni(CO)₄]^.+ salt as the first synthesis of any homoleptic nickel carbonyl cation in the condensed phase. It was prepared by oxidation of nickel metal with the synergistic oxidant Ag[F{Al(OR^F)₃}₂]/0.5 I₂ (R^F=C(CF₃)₃) in CO atmosphere. This D_2d‐symmetric metalloradical represents the last missing entry among the structurally characterized homoleptic carbonyl cations of Groups 6 to 11. Additionally, the nickel tricarbonyl‐nitrosyl cation [Ni(CO)₃(NO)]⁺ was obtained by usage of NO[F{Al(OR^F)₃}₂] and all products were fully characterized by means of IR, Raman, NMR/EPR, single crystal and powder XRD.

Not Guilty on Every Count: The "Non-Innocent" Nitrosyl Ligand in the Framework of IUPAC's Oxidation-State Formalism

Nitrosyl-metal bonding relies on the two interactions between the pair of N-O-π* and two of the metal's d orbitals. These (back)bonds are largely covalent, which makes their allocation in the course of an oxidation-state determination ambiguous. However, apart from M-N-O-angle or net-charge considerations, IUPAC's "ionic approximation" is a useful tool to reliably classify nitrosyl metal complexes in an orbital-centered approach.

Can We Safely Obtain Formal Oxidation States from Centroids of Localized Orbitals?

The use of centroids of localized orbitals as a method to derive oxidation states (OS) from first-principles is critically analyzed. We explore the performance of the closest-atom distance criterion to assign electrons for a number of challenging systems, including high-valent transition metal compounds, π-adducts, and transition metal (TM) carbenes. Here, we also introduce a mixed approach that combines the position of the centroids with Bader's atomic basins as an alternative criterion for electron assignment. The closest-atom criterion performs reasonably well for the challenging systems, but wrongly considers O-H and N-H bonds as hydrides. The new criterion fixes this problem, but underperforms in the case of TM carbenes. Moreover, the OS assignment in dubious cases exhibit undesirable dependence on the particular choice for orbital localization.

PtX as the limit of high oxidation states in oxide–nitride species

Neutral PtO₂N₂ and PtNO₃⁺ and PtN₃O⁻ ions are theoretically predicted to be sufficiently metastable to be prepared by a skillful experimenter in cryogenic conditions. PtNO₃⁺ has a longer lifetime than the previously claimed PtO₄²⁺.

σ-Noninnocence: Masked Phenyl-Cation Transfer at Formal NiIV

Reductive elimination is an elementary organometallic reaction step involving a formal oxidation state change of -2 at a transition-metal center. For a series of formal high-valent NiIV complexes, aryl-CF3 bond-forming reductive elimination was reported to occur readily (Bour et al. J. Am. Chem. Soc. 2015, 137, 8034-8037). We report a computational analysis of this reaction and find that, unexpectedly, the formal NiIV centers are better described as approaching a +II oxidation state, originating from highly covalent metal-ligand bonds, a phenomenon attributable to σ-noninnocence. A direct consequence is that the elimination of aryl-CF3 products occurs in an essentially redox-neutral fashion, as opposed to a reductive elimination. This is supported by an electron flow analysis which shows that an anionic CF3 group is transferred to an electrophilic aryl group. The uncovered role of σ-noninnocence in metal-ligand bonding, and of an essentially redox-neutral elimination as an elementary organometallic reaction step, may constitute concepts of broad relevance to organometallic chemistry.

Nine questions on energy decomposition analysis

The paper collects the answers of the authors to the following questions: Is the lack of precision in the definition of many chemical concepts one of the reasons for the coexistence of many partition schemes? Does the adoption of a given partition scheme imply a set of more precise definitions of the underlying chemical concepts? How can one use the results of a partition scheme to improve the clarity of definitions of concepts? Are partition schemes subject to scientific Darwinism? If so, what is the influence of a community's sociological pressure in the "natural selection" process? To what extent does/can/should investigated systems influence the choice of a particular partition scheme? Do we need more focused chemical validation of Energy Decomposition Analysis (EDA) methodology and descriptors/terms in general? Is there any interest in developing common benchmarks and test sets for cross-validation of methods? Is it possible to contemplate a unified partition scheme (let us call it the "standard model" of partitioning), that is proper for all applications in chemistry, in the foreseeable future or even in principle? In the end, science is about experiments and the real world. Can one, therefore, use any experiment or experimental data be used to favor one partition scheme over another? © 2019 Wiley Periodicals, Inc.

[Fe(H2 O)5 (NO)]2+ , the "Brown-Ring" Chromophore

Although the "brown-ring" ion, [Fe(H₂ O)₅ (NO)]²⁺ (1), has been a research target for more than a century, this poorly stable species had never been isolated. We now report on the synthesis of crystals of a salt of 1 which allowed us to tackle the unique bonding situation on an experimental basis. As a result of the bonding analysis, two stretched, spin-polarised π-interactions provide the Fe-NO binding-and challenge the concept of "oxidation state".