Metal-C02 complexes have attracted attention from both fundamental and applied viewpoints.1 First, C02 is an abundant Q molecule that serves as nature's photosynthetic building block.Second, C02 exhibits a variety of binding modes to metals.1"6 Third, the reactivity of metal-C02 complexes should provide insight regarding C02 activation and the design of new catalytic reactions.7 We have previously described the synthesis of the Scheme I. Representative Syntheses and Reactions of Rhenium/ Main-Group-Metal Bridging Carboxylate Complexes Ci c C MH R3SnCI /jv, ON/(TXPPh3

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Four new papers have appeared in print since the beginning of the project period (#76-79) on January 1, 1993. Principal achievements are as follows. .A_.______9__E_quilibria in Aldehyde and Ketone Complexes (pub. #76) Aldehydes are reactants, intermediates, or products in many metal-catalyzed reactions. We have sought to identify the factors that influence the thermodynamics of 7/o binding, and delineate differences in reactivity between the two binding modes.

September6, 1991 I. RESEARCHPROGRESS This has been an extremelyproductiveprojectperiod. Six new papers have appeared in print or been acceptedfor publicationsince the previousreport (A9-14). Principalachievementsare as follows. A. 7/0 Equilibriain Aldehyde and Ketone Complexes(pub.A-8t A-11) Aldehyde and ketone complexesare intermediates in many metal-catalyzed reactions. We have sought to define the fr.ctors that affect the thermodynamics of 7/0 binding,and differencesin reactivityof the two bindingnw}des. In the pI'evious report, we describedfifteen aromaticaldehydecomplexesof the form-5_C5H 5 ula [(n)Re(NO)(PPh3)(O=CHAr)] + BF4. IR analyses (CH2Cl 2) showed these to be >96:<4 to 15:85mixturesof 7/0 isomers (vNO: _ 1745-1733;o 1701-1692 cm-1). Electronwithdrawingsubstituer0ts (whichenhance aldehyde_ acidityand lower o basicity)favor the _ bindingmode, while electrondon6ting substituents have ar oppositeeffect. We wonder._d whether parallelelectroniceffectsmight be observablein ketone complexes. Methyl ketonecomplexes [(nS-c5H5)Re(NO) (PPh3)(nI-o=C(CH3)R) ]+ X-exist exclusivelyas o isomers. Thus, we sought to prepare analogswith electron-withdrawing ketonesubstituents. Accordingly,reactionsof dichloromethane c_,plex [(n5-CsH5)Re(NO) (PPh 3)(CICH2Cl) ]+ BF4" (I) with 1,3-dichloroacetoneand 1,3-difluoroacetone gave the _ complexes[(n5,C5H5)Re(NO)(PPh3)(n 2