The role of binuclear rhodium complexes as possible intermediates in the formation of cyclopentadienone-rhodium complexes. Reactions between (η-C5H5),Rh2(CO)(CF3C2CF3) and symmetrically disubstituted alkynes

A stepwise pathway for the formation of cyclopentadienone complexes (η-C5H5)Rh{C4R4CO} from (#-C5H5)Rh(CO)2 and RC≡CR has been elucidated. When solutions of (η-C5H5)Rh(CO)2 are heated, there is loss of CO and (η-C5H5),Rh2(CO)3 is formed. This reacts with alkynes to give binuclear complexes such as (η-C5H5),Rh2(CO)2(CF3C2CF3) gives (ηC5H5)2Rh2(CO)(ButC2But). Removal of CO from (η-C5H5)2Rh2(CO)2(CF3C2CF3) gives (η-C5H5)2Rh2(CO)(CF3C2CF3) and this monocarbonyl complex reacts with more alkyne, R'C≡CR', to form the bis-alkyne complex (η-C5H5)2Rh2(CO)(CF3C2CF3)(R'C2R')(R' = SiMe3) and then the pentadienone complex (η-C5H5)2Rh2{C4(CF3)2R'2CO} (R' = =Me, Me3Si, CF3). In some instances (R' = Ph, But, CO2Me), this reaction gives equimolar amounts of the metallodiene complex (η-C5H5)2Rh2{C4{CF3)2R'2} plus the dicarbonyl complex (η-C5H,)2Rh2(CO)2(CF3C2CF3). The binuclear pentadienone complexes (η-C5H5),Rh2{C4R2R'2CO) (R = CF, and R' = Me; or R = R' = Et) can be converted into mononuclear cyclopentadienone complexes (η-C5H5)Rh{C4R2R'2CO} by heating in the presence of CO or alkyne, or both.