PHOTOCHEMISTRY OF MONOSUBSTITUTED MANGANESE CARBONYL-COMPOUNDS, MN-2(CO)(9)L, IN 3-METHYLPENTANE AT LOW-TEMPERATURE - FLASH-PHOTOLYSIS STUDIES IN HEXANE AT ROOM-TEMPERATURE

The photochemical behavior of monosubstituted manganese carbonyl dimer s, Mn-2(CO)(9)L (L =PMe(3), P(n-Bu)(3), PPh(3), P(CH(2)Ph)(3), and P(i -Pr)(3)), in 3-methylpentane at 93 K has been studied. Loss of CO lead s to formation of the semibridging structure Mn-2(CO)(7)L(mu-eta(1),et a(2)CO). Because loss of CO might occur from either of the two nonequi valent metals, and because the phosphine exhibits positional isomerism , several isomeric semibridging structures are possible, depending on the steric requirements of the phosphine. Each gives rise to a unique semibridging CO stretching frequency. Upon warming of the low-temperat ure glass, recombination with CO and intramolecular rearrangements of the semibridging forms occur in competition. Recombination with CO occ urs most readily at the least sterically hindered sites. Thus, recombi nation to form the parent carbonyl, Mn-2(CO)(9)L, occurs most readily with Mn-2(CO)(9)PMe(3) and most slowly with Mn-2(CO)(9)P(i-Pr)(3). Stu dies of the PMe(3) and PPh(3) complexes show that recombination with C O leads to substantial quantities of the equatorial form, which isomer izes to the axial form as the solution is warmed. At room temperature the thermodynamic equilibrium between the equatorial and axial isomers of Mn-2(CO)(9)L lies heavily toward the axial form. Flash photolysis of hexane solutions of Mn-2(CO)(9)L compounds in hexane solutions at r oom temperature reveal a complex decay of absorbances due to CO loss p roducts. Analysis of the transient behavior leads to. three pseudo-fir st-order rate constants: one is due to recombination of CO with Mn-2(C O)(9), formed via photochemical loss of L. The other two correspond to recombination of CO with Mn-2(CO)(8)L intermediates in which the CO h as been lost from either Mn center.