Spin triplet (PNP)Co adds one CO according to a rate law first-order in each reagent with k = 9.3 x 105 M-1 s(-1). In contrast, the transient produced by photodissociation of CO from (PNP)Rh(CO) forms, within 10 ns, the Rh(III) species [tau](4)t Bu2PCH2SiMe2NSiMe2CH2PtBu(CMe2CH2)]RhH, and this adds CO with a second-order rate constant of 6 x 106 M-1 s-1. The resulting monocarbonyl then slowly (k = 3 x 10(1) s(-1)) reductively eliminates H and C(sp(3)) to re-form (PNP)Rh(CO). The former two rate constants support the idea that a spin crossing between triplet (PNP)Co and singlet (PNP)Co(CO) causes a rate suppression.