Thermodynamic, Kinetic, and Computational Study of Heavier Chalcogen (S, Se, and Te) Terminal Multiple Bonds to Molybdenum, Carbon, and Phosphorus

Enthalpies of chalcogen atom transfer to Mo(N[<i>t</i>-Bu]Ar)₃, where Ar = 3,5-C₆H₃Me₂, and to IPr (defined as <i>bis</i>-(2,6-isopropylphenyl)imidazol-2-ylidene) have been measured by solution calorimetry leading to bond energy estimates (kcal/mol) for EMo(N[<i>t</i>-Bu]Ar)₃ (E = S, 115; Se, 87; Te, 64) and EIPr (E = S, 102; Se, 77; Te, 53). The enthalpy of S-atom transfer to PMo(N[<i>t</i>-Bu]Ar)₃ generating SPMo(N[<i>t</i>-Bu]Ar)₃ has been measured, yielding a value of only 78 kcal/mol. The kinetics of combination of Mo(N[<i>t</i>-Bu]Ar)₃ with SMo(N[<i>t</i>-Bu]Ar)₃ yielding (μ-S)[Mo(N[<i>t</i>-Bu]Ar)₃]₂ have been studied, and yield activation parameters Δ<i>H</i>^‡ = 4.7 ± 1 kcal/mol and Δ<i>S</i>^‡ = −33 ± 5 eu. Equilibrium studies for the same reaction yielded thermochemical parameters Δ<i>H</i>° = −18.6 ± 3.2 kcal/mol and Δ<i>S</i>° = −56.2 ± 10.5 eu. The large negative entropy of formation of (μ-S)[Mo(N[<i>t</i>-Bu]Ar)₃]₂ is interpreted in terms of the crowded molecular structure of this complex as revealed by X-ray crystallography. The crystal structure of Te-atom transfer agent TePCy₃ is also reported. Quantum chemical calculations were used to make bond energy predictions as well as to probe terminal chalcogen bonding in terms of an energy partitioning analysis.