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

The photochemistry of Mo(CO)(6) in heptane has been investigated using actinometry and photoacoustic calorimetry. The quantum yield for CO s ubstitution on Mo(CO)(6) by piperidine in heptane was found to be 0.93 for 337 nm irradiation at 25 degrees C. This was clearly different th an the quantum yield found for CO substitution on Cr(CO)(6) by piperid ine (0.75). Two heat decays were observed following flash photolysis o f Mo(CO)(6) in the presence of piperidine. The first heat decay is ind ependent of piperidine concentration (8-80 mM), while the lifetime (ta u(2)) of the second heat decay decreases with increasing piperidine co ncentration. A plot of 1/tau(2) shows a first-order dependence on pipe ridine concentration. The first heat decay has been assigned to the di splacement of CO on Mo(CO)(6) by heptane and the second, to the displa cement of heptane on Mo(CO)(5)(heptane) by piperidine. The second-orde r rate constant for heptane displacement is 4.8 x 10(7) M(-1)s(-1). Th e enthalpies of CO displacement by heptane (Delta H-1) and heptane dis placement by piperidine (Delta H-2) are estimated as 24 and -17 kcal/m ol, respectively. This yields an enthalpy of CO substitution by piperi dine (Delta H-1 + Delta H-2) of 6.3 kcal/mol in agreement with literat ure results. The Mo(CO)(5),(heptane) and Mo(CO)(5) (piperidine) bond d issociation energies are estimated to be 17 and 34 kcal/mol, respectiv ely. The former value is more likely to be affected by corrections for reaction volume changes and should be treated as an upper limit.