The rate of reaction of hydrogen with the 17 e - metal centered radical * Cr(CO) 3 C 5 Me 5 obeys the third-order rate law d[P]/dt = k o b s [ * Cr] 2 [H 2 ] in toluene solution. In the temperature range 20-60 o C, k o b s = 330 +/- 30 M - 2 s - 1 , ΔH < > = 0 +/- 1 kcal mol - 1 , ΔS < > = - 47 +/- 3 cal mol - 1 deg - 1 . The rate of oxidative addition is not inhibited by added pressure of CO. The rate of binding of D 2 is slower than that of H 2 : k(H 2 )/k(D 2 ) = 1.18. These results are combined with earlier work to generate a complete reaction profile for hydrogenation of the metal-metal bonded dimer [Cr(CO) 3 C 5 Me 5 ] 2 + H 2 -> 2H-Cr(CO) 3 C 5 Me 5 . A similar reaction profile for Co 2 (CO) 8 + H 2 -> 2H-Co(CO) 4 under high pressures of CO is constructed based on literature data and estimated activation parameters for reaction of the * Co(CO) 4 radical with hydrogen.