Bond Energies, Reaction Volumes, and Kinetics for sigma - and pi -Complexes of Mo(CO) sub(5)L

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2011-08, Vol.115 (32), p.9004-9013-9004-9013
• Main Authors: Gittermann, Shannon M, Letterman, Roger G, Jiao, Tianjie, Leu, Ging-Long, DeYonker, Nathan J, Webster, Charles Edwin, Burkey, Theodore J
• Format: Article
• Language: English
• Subjects: Alkanes; Bond energy; Computation; Lewis base; Mathematical analysis; Molybdenum; Rate constants; Solvents
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp203915q

The photosubstitution reactions of molybdenum hexacarbonyl with sigma and pi donor ligands were investigated using photoacoustic calorimetry and computational methods in a series of linear alkane solvents (pentane, hexane, heptane, octane, decane, and dodecane). The results show that reaction volumes make a significant contribution to the photoacoustic signal and must be considered during thermodynamic calculations based on photoacoustic measurements. The enthalpies of CO substitution by an alkane solvent and subsequent substitution by each Lewis base were determined. Corresponding Mo-L bond energies (kcal mol super(-1)) were calculated: L = linear alkanes (13), triethylsilane (26), 1-hexyne (27), 1-hexene (27), and benzene (17). The relative energies are in agreement with computational results. The experimental reaction volume for CO substitution by alkane was positive (15 mL mol super(-1)) and negative or close to zero for alkane substitution by a Lewis base (for example, -11 mL mol super(-1) for triethylsilane and 3.6 mL mol super(-1) for benzene). The errors in the experimental and computational reaction volumes are large and often comparable to the reaction volumes. An improved calibration of the methods as well as a better understanding of the underlying physics involved is needed. For the Lewis bases reported in this study, the second-order rate constants for the displacement of a coordinated alkane are less than diffusion control (5 10 super(6)-4 10 super(7) M super(-1) s super(-1)) and decrease monotonically with the alkane chain length. The rate constants correlate better with steric effects than with bond energies. An interchange mechanism is consistent with the results.