Computational methods
Density functional theory (DFT) methods belonging to the hybrid GGA class were used in the calculations. The geometry of the compounds was optimized with the PBE0 functional, which was successfully applied for similar investigations of molybdenum complexes on the basis of test calculations involving various DFT methods. The triple-ζ valence def2-TZVPP basis set, recommended for quantitative DFT and Hartree-Fock calculation, was applied for all the elements. The 28 and 60 innermost electrons of Mo and W, respectively, were replaced by the Stuttgart effective core potential. Thermochemical quantities (T=298.15 K) were evaluated in the rigid-rotor and harmonic-oscillator approximations.