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Tetra-(3,5 dimethylpheny1)porphyrinato rhodium(ii)dimer, [(TXP)RhI2, reacts reversibly with CO to form a 1,2-ethanedionyl complex, (TXP)Rh-C(O)C(O)-Rh(TXP), which constitutes the first well-characterized example of reductive coupling of CO to form a dimetal a-diketone species.Development of strategies to promote reductive coupling of carbon monoxide as a pathway for C-C bond formation continues as a major interest of organometallic-catalysis research.l-6 Metal complex induced reductive coupling of CO can result in two limiting types of coupling products, acetylene diolates, M-O-CrC-O-M, and 172-ethanedionyl species, M-C(0)-C(0)-M [equations ( 1) and ( 2)]. Alkali metals are known to reduce CO to form ionic acetylene diolates,7 and we have recently observed that octaethylporphyrin rhodium dimer, [(OEP)Rh]2, reacts with CO to produce an equilibrium distribution with three CO containing species, a mono CO adduct, [(OEP)Rh]2C07 a dimetal ketone, (0EP)Rh-C(0)-Rh( OEP) , and a species formulated as a 1 ,Zethanedionyl complex, (0EP)Rh-C(0)-C(0)-Rh(0EP) (1).8 The pro-

Benzene solutions of [(TMTAA)Rh](2) (1) react with CO (P(CO) = 0.8-20 atm; T = 298 K) by cleaving the Rh(II)-Rh(II) bond to form dirhodium(III) ketone (TMTAA)Rh-C(O)-Rh(TMTAA) [2; ν(CO) = 1726 cm(-1); (1)J(103)Rh(13)C(O)(103)Rh = 45 Hz]. Thermodynamic values for the reaction of 1 with CO to form 2 were evaluated from equilibrium constant measurements [K(1)(298 K) = 5.0(0.6) × 10(3), ΔG(1)°(298 K) = -5.0(0.1) kcal mol(-1), ΔH(1)° = -14(1) kcal mol(-1), and ΔS(1)° = -30(3) cal K(-1) mol(-1)].

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