Analysis of chemical bonding of the ground and low-lying states of Mo and of MoCl 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 XΣ state of Mo have been extrapolated to the complete basis set limits. Our final values, r = 1.9324 Å and D (D) = 4.502 ± 0.007(4.471 ± 0.009) eV, are in excellent agreement with the experimental values of r = 1.929, 1.938(9) Å and D = 4.476(10) eV. Mo in the Σg+13 state is a weakly bound dimer, forming 5s⋯5p bonds, with D = 0.120 eV at r = 3.53 Å. All calculated excited states (except Σg+13) have a highly multireference character (C = 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 Δ 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 Mo, as the multiplicity of spin is increased, is analyzed in parallel with the Mo-Mo bond breaking in a series of MoCl complexes when x is increased. Physical insight into the nature of the sextuple bond and its low dissociation energy is provided.