SYNTHESIS OF M2RH2 BIS(MU(3) CARBON-DIOXIDE) COMPLEXES FROM THE REACTION BETWEEN [RH(OH)(ETA(4)-COD)](2) AND CATIONIC METAL-CARBONYLS

The M2Rh2 bis(mu(3)-CO2) complexes [Cp(CO)(NO)Re(CO2)Rh(eta(4)-Cod)]( 2) (1) and [Cp(CO)(2)-MCO2Rh(eta(4)-cod)](2), M = Fe (2) and Ru (3), were synthesized in moderate to high yields from treating [Rh(OH)(eta( 4)-cod)](2) (cod = 1,5-cyclooctadiene) with the carbonyl salts Cp(L)( CO)(2)M+ (L = CO, M = Fe, Ru; L = NO, M = Re) and a base. [An alternat ive synthesis of 1 and its crystallographic structure determination ha s been reported.] Although details on using several bases in the synth esis of 2 are reported, the use of volatile EtMe2N in excess as the ba se is especially useful. IR and NMR spectral data are in accord with a M2Rh2 bis(mu(3)-CO2) core for 2 and 3: they retain two mu(3)-[eta(1)- C(M):eta(1)-O(Rh):eta(1)-O'(Rh')] carboxylate ligands that resemble th ose that were found for 1 and the Rh(I) carboxylates [(RCO2)Rh(diene)] (2). Complexes 2 and 3 with their ''open-book'' structures are not flu xional at room temperature. Complex 2, its norbornadiene analogue, [Cp (CO)(2)FeCO2Rh(eta(4)-nbd)](2) (4), and their C-13-labeled derivative s also are accessible from reactions of Cp(CO)(2)FeCO2K [or Cp*(CO)(2 )(FeCO2K)-C-13] with [Rh(OSO2CF3)(eta(4)-cod)](x). [Cp(CO)(2)Fe((CO2) -C-13)Rh(eta(4)-diene)](2) [diene = cod (2-C-13); nbd (4-C-13)] underw ent carboxylate -carbonyl label shuttle to yield 1:1 mixtures of [Cp( CO)((CO)-C-13)FeCO2Rh(diene)](2) (2a-C-13) and 4a-C-13. IR spectral as signments for the metallocarboxylate upsilon(oco) absorptions are also presented.