Since the discovery of a formal quintuple bond in Ar′CrCrAr′ (CrCr = 1.835 Ã…) by Power and co-workers in 2005, many efforts have been dedicated to isolating dichromium species featuring quintuple-bond character. In the present study we investigate the electronic configuration of several, recently synthesized dichromium species with ligands using nitrogen to coordinate the metal centers. The bimetallic bond distances of Power’s compound and Cr₂-diazadiene (1) (CrCr = 1.803 Ã…) are compared to those found for Cr₂(μ-η²-ArNC(R)NAr)₂ (2) (CrCr = 1.746 Ã…; R = H, Ar = 2,6-Et₂C₆H₃), Cr₂(μ-η²-Ar^XylNC(H)NAr^Xyl)₃ (3) (CrCr = 1.740^reduced/1.817^neutral Ã…; Ar^Xyl= 2,6-C₆H₃-(CH₃)₂), Cr₂(μ-η²-TippPyNMes)₂ (4) (CrCr = 1.749 Ã…; TippPyNMes = 6-(2,4,6-triisopropylphenyl)pyridin-2-yl (2,4,6-trimethylphenyl)amide), and Cr₂(μ-η²-DippNC(NMe₂)N-Dipp)₂ (5) (CrCr = 1.729 Ã…, Dipp = 2,6-i-Pr₂C₆H₃). We show that the correlation between the CrCr bond length and the effective bond order (EBO) is strongly affected by the nature of the ligand, as well as by the steric hindrance due to the ligand structure (e.g., the nature of the coordinating nitrogen). A linear correlation between the EBO and CrCr bond distance is established within the same group of ligands. As a result, the CrCr species based on the amidinate, aminopyridinate, and guanidinate ligands have bond patterns similar to the Ar′CrCrAr′ compound. Unlike these latter species, the dichromium diazadiene complex is characterized by a different bonding pattern involving Cr−NÏ€ interactions, resulting in a lower bond order associated with the short metal−metal bond distance. In this case the short CrCr distance is most probably the result of the constraints imposed by the diazadiene ligand, implying a Cr₂N₄ core with a closer CrCr interaction.

The results of a computational study with multiconfigurational quantum chemical methods on actinide monoxides (AnO) and dioxides (AnO₂) for An = Th, Pa, U, Np, Pu, Am, and Cm, are presented. First and second ionization energies were determined and compared with experimental values, when available. The trend along the series is analyzed in terms of the electronic configurations of the various species. The agreement with experiment is excellent in most cases. Of particular interest is the first ionization of PuO₂. We applied cutting-edge theoretical methods to refine the ionization energy, but our computed data fall in the range of ~6 eV and not in the ~7 eV region as the experiment dictates. Such a system requires further computational and experimental attention.

CoCo loco! Ligand-bridged dimers (see picture) with the shortest known Co-Co interactions are the first amidinato and guanidinato cobalt(I) complexes. The nature of the interactions has been probed by magnetic and theoretical investigations, and has been shown to be multiconfigurational. Preliminary reactivity studies of the complexes have also been carried out.

Multiconfigurational quantum chemical calculations on the R-diimines dichromium compound confirm that the Cr−Cr bond, 1.80 Å, is among the shortest Cr^I−Cr^I bonds. However, the bond between the two Cr atoms is only a quadruple bond rather than a quintuple bond. The reason why the bond is so short has to be attributed to the strain in the NCCN ligand moieties.

The ground and excited states of neutral and cationic PuO and PuO₂ have been studied with multiconfigurational quantum chemical methods followed by second order perturbation theory, the CASSCF/CASPT2 method. Scalar relativistic effects and spin–orbit coupling have been included in the treatment. As literature values for the ionization energy of PuO₂ are in the wide range of ~6.6 eV to ~10.1 eV, a central goal of the computations was to resolve these discrepancies; the theoretical results indicate that the ionization energy is near the lower end of this range. The calculated ionization energies for PuO, PuO⁺ and PuO₂⁺ are in good agreement with the experimental values.

Quantum mechanical calculations, using both CASPT2 and DFT methods, for the model systems (MeMMMe, PhMMPh, (MeMMMe)(C₆H₆)₂, Ar^§MMAr^§, Ar^#MMAr^#; M = Cr, Fe, Co; Ar^§ = C₆H₄-2(C₆H₅), Ar^# = C₆H₃-2,6(C₆H₃-2,6-Me₂)₂) are described. These studies were undertaken to provide a multireference description of the metal−metal bond in the simple dimers MeMMMe and PhMMPh (M = Cr, Fe, Co) and to determine the extent of secondary metal−arene interaction involving the flanking aryl rings of the terphenyl ligands in quintuply bonded Ar′CrCrAr′ (Ar′ = C₆H₃-2,6(C₆H₃-2,6-Prⁱ₂)₂). We show that in the Cr−Cr species the Cr−arene interaction is a feeble one that causes only a small weakening of the quintuple bond. In sharp contrast, in the analogous Fe and Co species strong η⁶-arene interactions that preclude significant metal−metal bonding are predicted.

Values of σ and σ⁺, for use in linear free energy relationships, are determined for para hydrogen atoms having nuclear charges other than 1 (nucleomers). Hammett Ï� values for a variety of free energies of activation, reaction, and other extrathermodynamic properties (e.g., vibrational frequencies) are computed therefrom and compared to those computed using typical para functional groups. The nucleomer correlations show excellent qualitative agreement with standard correlations but the quantitative agreement is less good, typically underestimating the standard Ï�-value by 10-60%.

Nice to see U₂! The [PhUUPh] molecule (see picture; U pink, C gray, H white) has been studied by multiconfigurational quantum chemical methods. It was found that a quintuple bond is formed between the two uranium atoms with a U—U bond length of 2.29 Ã…. The phenyl ligand was used to mimic a bulky terphenyl ligand, which could be a promising candidate for the stabilization of multiply bonded uranium compounds.