ALKANE COORDINATION BY MOLYBDENUM AND CHROMIUM PENTACARBONYLS: AN EXAMINATION OF THE ENERGETICS OF INTERMOLECULAR AGOSTIC BONDING

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Abstract
The photochemistry of Mo(CO)6 in heptane has been investigated using actinometry and photoacoustic calorimetry. The quantum yield for CO substitution on Mo(CO)6 by piperidine in heptane was found to be 0.93 for 337 nm irradiation at 25°C This was clearly different than the quantum yield found for CO substitution on Cr(CO)6 by piperidine (0.75). Two heat decays were observed following flash photolysis of Mo(CO)6 in the presence of piperidine. The first heat decay is independent of piperidine concentration (8–80 mM), while the lifetime (τ 2 ) of the second heat decay decreases with increasing piperidine concentration. A plot of 1/τ2 shows a first-order dependence on piperidine concentration. The first heat decay has been assigned to the displacement of CO on Mo(CO)6 by heptane and the second, to the displacement of heptane on Mo(CO)5(heptane) by piperidine. The second-order rate constant for heptane displacement is 4.8 × 107 M−1S−1. The enthalpies of CO displacement by heptane (ΔH 1) and heptane displacement by piperidine (ΔH 2) are estimated as 24 and −17 kcal/mol, respectively. This yields an enthalpy of CO substitution by piperidine (ΔH 1 + ΔH 2) of 6.3 kcal/mol in agreement with literature results. The Mo(CO)5, (heptane) and Mo(CO)5 (piperidine) bond dissociation energies are estimated to be 17 and 34 kcal/mol, respectively. The former value is more likely to be affected by corrections for reaction volume changes and should be treated as an upper limit.

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