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The angular overlap model (AOM) is an established parameterization scheme within ligand field theory (LFT). In principle, its application is fairly straightforward, but can be tedious and involve a trial‐and‐error approach to identify and judge the best set of parameters. With the availability of quantum chemical methods to predict d‐d transitions in transition metal complexes, a rich source of computational spectroscopic data with unambiguous assignments to electronic states is available. Herein, we present AOMadillo, a software package that is designed to interface the output of ab initio LFT calculations from the ORCA suite of programs and performs a least‐squares fit for a chosen AOM parameterization. Many steps of the AOM parameterization are automated, so that scans of geometric parameters and evaluations of sets of similar complexes are convenient. The fitting routine is highly configurable, allowing the efficient evaluation of different parameter sets. [ABSTRACT FROM AUTHOR]

The sustainable fixation of atmospheric N2 and its conversion into industrially relevant molecules is one of the major current challenges in chemistry. Besides nitrogen activation with transition metal complexes, a "push‐pull" approach that fine‐tunes electron density along the N−N bond has shown success recently. The "pushing" is performed by an electron rich entity such as a transition metal complex, and the "pulling" is achieved with an electron acceptor such as a Lewis acid. In this contribution, we explore the electronic structure implications of this approach using the complex trans‐[ReICl(N2)(PMe2Ph)4] as a starting point. We show that borane Lewis acids exert a pull‐effect of increasing strength with increased Lewis acidity via a π‐pathway. Furthermore, the ligand trans to dinitrogen can weaken the dinitrogen bond via a σ‐pathway. Binding a strong Lewis acid is found to have electronic structure effects potentially relevant for electrochemistry: dinitrogen‐dominated molecular orbitals are shifted into advantageous energetic positions for redox activation of the dinitrogen bond. We show how these electronic structure design principles are rooted in cooperative effects of a transition metal complex and a Lewis acid, and that they can be exploited to tailor a complex towards the desired thermal, electrochemical or photochemical reactivity. [ABSTRACT FROM AUTHOR]

The properties and reactivities of transition metal complexes are often discussed in terms of Ligand Field Theory (LFT), and with ab initio LFT a direct connection to quantum chemical wavefunctions was recently established. The Angular Overlap Model (AOM) is a widely used, ligand‐specific parameterization scheme of the ligand field splitting that has, however, been restricted by the availability and resolution of experimental data. Using ab initio LFT, we present here a generalised, symmetry‐independent and automated fitting procedure for AOM parameters that is even applicable to formally underdetermined or experimentally inaccessible systems. This method allows quantitative evaluations of assumptions commonly made in AOM applications, for example, transferability or the relative magnitudes of AOM parameters, and the response of the ligand field to structural or electronic changes. A two‐dimensional spectrochemical series of tetrahedral halido metalates ([MIIX4]2−, M=Mn−Cu) served as a case study. A previously unknown linear relationship between the halide ligands' chemical hardness and their AOM parameters was found. The impartial and automated procedure for identifying AOM parameters introduced here can be used to systematically improve our understanding of ligand–metal interactions in coordination complexes. [ABSTRACT FROM AUTHOR]

While the thermal activation of the dinitrogen molecule has been demonstrated in numerous examples with a variety of transition metals and ligand frameworks, the use of light to induce a weakening or splitting of the strong N–N bond is less well explored. Six complexes that bind N2 in a linear μ–η1:η1-end-on fashion between two transition metals are known to cleave dinitrogen after absorbing a photon and relaxing from an electronically excited state. This Perspective article reviews the molecular complexes known to be capable of dinitrogen photocleavage, and discusses mechanistic insights into the photoactivation process gained from experimental and computational studies. In an extension of previous hypotheses for pathways to dinitrogen photoactivation that would facilitate easy protonation of the μ-N atoms, a scheme is presented that may help to identify other complexes that could be capable of dinitrogen photoactivation. [ABSTRACT FROM AUTHOR]

Abstract: The search for molecular catalysts that efficiently activate or cleave the dinitrogen molecule is an active field of research. While many thermal dinitrogen cleavage catalysts are known, the photochemical activation of N2 has received considerably less attention. In this paper, the first computational study of the osmium dimer [Os(II,III)2(μ‐N2)(NH3)10]5+, which was shown to be capable of dinitrogen photocleavage, is presented. Despite its deceptively simple geometry, it has a complex electronic structure with a valence‐delocalized and electronically degenerate ground state. Using multiconfigurational methods, the electronic structure at the ground state geometry and along the dinitrogen cleavage coordinate was investigated. The results indicate that an unoccupied molecular orbital with σ‐bonding character between osmium and μ‐N atoms and σ‐antibonding dinitrogen character is most affected by N–N distance elongation. This implies that a lower barrier for thermal or photochemical N2 activation in linear M‐N‐N‐M complexes can be achieved by lowering the energetic separation between this unoccupied orbital and the HOMO, representing a specific target for future catalyst design. [ABSTRACT FROM AUTHOR]

A frequent challenge when dealing with multinuclear transition metal clusters in biology is to determine the absolute oxidation states of the individual metal ions and to identify how they evolve during catalytic turnover. The oxygen-evolving complex of biological photosynthesis, an active site that harbors an oxo-bridged Mn4Ca cluster as the water-oxidizing species, offers a prime example of such a challenge that withstood satisfactory resolution for decades. A multitude of experimental studies have approached this question and have offered insights from different angles, but they were also accompanied by incomplete or inconclusive interpretations. Only very recently, through a combination of experiment and theory, has a definitive assignment of the individual Mn oxidation states been achieved for all observable catalytic states of the complex. Here we review the information obtained by structural and spectroscopic methods, describe the interpretation and synthesis achieved through quantum chemistry, and summarize our current understanding of the electronic structure of nature's water splitting catalyst. [ABSTRACT FROM AUTHOR]