Activation parameters in flash photolysis studies of Mo(CO)6

Formation and reactivity of organometallic alkane complexes

“…Complexes of the type M(CO) 5 (L) (M = Cr, Mo, W; L = alkane) have been observed following the excitation of M(CO) 6 by UV photolysis [24][25][26] and by pulse radiolysis [27] in both solution and in the gas phase at room temperature. Ultrafast transient absorption experiments have demonstrated that Cr(CO) 5 (cyclohexane) is formed within 1 ps, following the photolysis of Cr(CO) 6 in a room temperature cyclohexane solution [28].…”

“…From these kinetic experiments an estimate of the W-alkane bond energy of 16 kJ mol −1 was reported [29]. A series of TRIR experiments have also examined the activation parameters for the reaction of M(CO) 5 (alkane) complexes with competing ligands [24][25][26]. Metal alkane binding energies have previously been determined by photoacoustic calorimetry (PAC) for a range of Group VI metal alkane complexes including: Cr(CO) 5 (n-heptane) (46 (±4) kJ mol −1 ) [30], Cr(benzene)(CO) 2 (n-heptane) (ca.…”

Bond Energies, Reaction Volumes, and Kinetics for σ- and π-Complexes of Mo(CO)5L

“…Unfortunately, the magnitudes of many metal–ligand bond energies are not well established . Many bond energies determined in solution are based on activation enthalpies of ligand dissociation; however, ligand dissociation in solution often involves the solvent in the transition state and the extent of solvation may be unclear . In general, accurate bond energies, either experimental or computational, are needed to understand and predict the bond energy difference required for selective bond cleavage.…”

“…Ultrafast solvent coordination can occur even for solvents as weakly basic as alkanes . Thermal relaxation of the metal-alkane complex is complete in a few hundred picoseconds, , and depending on the concentration of the Lewis base, L , the second step is typically on the microsecond time scale , where the observed rate constant ( k obs ) is directly related to the microscopic rate constant k (eq ). Following a laser pulse, PAC can resolve the acoustic waves of the faster processes of the first step from those of the second step.…”

“…The results are used to determine the enthalpy of each step and the rate constant for the second step. d false[ Mo false( CO false) 5 L false] d t = k obs false[ Mo false( CO false) 5 .25em alkane false] = k false[ L false] false[ Mo false( CO false) 5 .25em alkane false] Because the differences in the energies of bonds that are broken and formed determine the enthalpy of each step, several bond energies can be calculated from the enthalpies of a single reference bond energy (in this case from Mo–CO). A number of studies have presented gas-phase (experimental and computational) and solution-phase (experimental , ) strengths of metal–alkane bonds. Previously, we reported results for Mo–alkane bond energies as well as those for ligands that bond via lone pairs. , Currently, we present PAC results for π- or σ-complexes with 1-hexyne, 1-hexene, benzene, triethylsilane, and a series of linear alkanes ( n -pentane, n -hexane, n -heptane, n -octane, n -decane, and n -dodecane).…”