Quadruple bonds in MoC: Accurate calculations and precise measurement of the dissociation energy of low-lying states of MoC.

In the present work, the electronic structure and chemical bonding of the MoC X3Σ- ground state and the six lowest excited states, A3Δ, a1Γ, b5Σ-, c1Δ, d1Σ+, and e5Π, have been investigated in detail using multireference configuration interaction methods and basis sets, including relativistic effective core potentials. In addition, scalar relativistic effects have been considered in the second order Douglas-Kroll-Hess approximation, while spin-orbit coupling has also been calculated. Five of the investigated states, X3Σ-, A3Δ, a1Γ, c1Δ, and d1Σ+, present quadruple σ2σ2π2π2 bonds. Experimentally, the predissociation threshold of MoC was measured using resonant two-photon ionization spectroscopy, allowing for a precise measurement of the dissociation energy of the ground state. Theoretically, the complete basis set limit of the calculated dissociation energy with respect to the atomic ground state products, including corrections for scalar relativistic effects, De(D0), is computed as 5.13(5.06) eV, in excellent agreement with our measured value of D0(MoC) of 5.136(5) eV. Furthermore, the calculated dissociation energies of the states having quadruple bonds with respect to their adiabatic atomic products range from 6.22 to 7.23 eV. The excited electronic states A3Δ2 and c1Δ2 are calculated to lie at 3899 and 8057 cm-1, also in excellent agreement with the experimental values of DaBell et al., 4002.5 and 7834 cm-1, respectively.

Potential of Molecular Catalysts with Electron-Rich Transition Metal Centers for Addressing Long-Standing Chemistry Enigmas.

Molecular complexes with electron-rich metal centers are highlighted as potential catalysts for the following five important chemical transformations: selective conversion of methane to methanol, capture and utilization of carbon dioxide, fixation of molecular nitrogen, water splitting, and recycling of perfluorochemicals. Our initial focus lies on negatively charged metal centers and ligands that can stabilize anionic metal atoms. Catalysts with electron-rich metal atoms (CERMAs) can sustain catalytic cycles with a "ping-pong" mechanism, where one or more electrons are transferred from the metal center to the substrate and back. The donated electrons can activate the chemical bonds of the substrate by populating its antibonding orbitals. At the last step of the catalytic cycle, the electrons return to the metal and the product interacts only weakly with the formed anion, which enables the solvent molecules to remove the product fast from the catalytic cycle and prevent subsequent unfavorable reactions. This process resembles electrocatalysis, but the metal serves as both an anode and a cathode (molecular electrocatalysis). We also analyze the usage of CERMAs as the base of Frustrated Lewis pairs proposing a new type of bimetallic catalysts. This Featured Article aspires to initiate systematic experimental and theoretical studies on CERMAs and their reactivity, the potential of which has probably been underestimated in the literature.

Electronic Structure and Chemical Bonding of the First-, Second-, and Third-Row-Transition-Metal Monoborides: The Formation of Quadruple Bonds in RhB, RuB, and TcB.

Boron presents an important role in chemistry, biology, and materials science. Diatomic transition-metal borides (MBs) are the building blocks of many complexes and materials, and they present unique electronic structures with interesting and peculiar properties and a variety of bonding schemes which are analyzed here. In the first part of this paper, we present a review on the available experimental and theoretical studies on the first-row-transition-metal borides, i.e., ScB, TiB, VB, CrB, MnB, FeB, CoB, NiB, CuB, and ZnB; the second-row-transition-metal borides, i.e., YB, ZrB, NbB, MoB, TcB, RuB, RhB, PdB, AgB, and CdB; and the third-row-transition-metal borides, i.e., LaB, HfB, TaB, WB, ReB, OsB, IrB, PtB, AuB, and HgB. Consequently, in the second part, the second- and third-row MBs are studied via DFT calculations using the B3LYP, TPSSh, and MN15 functionals and, in some cases, via multi-reference methods, MRCISD+Q, in conjunction with the aug-cc-pVQZ-PPM/aug-cc-pVQZB basis sets. Specifically, bond distances, dissociation energies, frequencies, dipole moments, and natural NPA charges are reported. Comparisons between MB molecules along the three rows are presented, and their differences and similarities are analyzed. The bonding of the diatomic borides is also described; it is found that, apart from RhB(X1Σ+), which was just recently found to form quadruple bonds, RuB(X2Δ) and TcB(X3Σ-) also form quadruple σ2σ2π2π2 bonds in their X states. Moreover, to fill the gap existing in the current literature, here, we calculate the TcB molecule.

Analysis of chemical bonding of the ground and low-lying states of Mo2 and of Mo2Clx complexes, x = 2-10.

In this study, we perform accurate calculations via multireference configuration interaction and coupled cluster methodologies on the dimolybdenum molecule in conjunction with complete series of correlation and weighted core correlation consistent basis sets up to quintuple size. The bonding, the dissociation energies, and the spectroscopic parameters of the seven states that correlate with the ground state products are calculated. The ground state has a sextuple chemical bond, and each of the calculated excited states has one less bond than the previous state. The calculated values for the ground X1Σg + state of Mo2 have been extrapolated to the complete basis set limits. Our final values, re = 1.9324 Å and De (D0) = 4.502 ± 0.007(4.471 ± 0.009) eV, are in excellent agreement with the experimental values of re = 1.929, 1.938(9) Å and D0 = 4.476(10) eV. Mo2 in the Σg+13 state is a weakly bound dimer, forming 5s⋯5pz bonds, with De = 0.120 eV at re = 3.53 Å. All calculated excited states (except Σg+13) have a highly multireference character (C0 = 0.25-0.55). The ordering of the molecular bonding orbitals changes as the spin is increased from quintet to septet state resulting in a change in energy separation ΔS,S-1 of the calculated states. The quite low bond dissociation energy of the ground state is due to the splitting of the molecular bonding orbitals in two groups differing in energy by ∼3 eV. Finally, the bond breaking of Mo2, as the multiplicity of spin is increased, is analyzed in parallel with the Mo-Mo bond breaking in a series of Mo2Clx complexes when x is increased. Physical insight into the nature of the sextuple bond and its low dissociation energy is provided.