Photolysis of BzCr(CO)3 and TpMn(CO)3 in the presence of H3B·NEt3 yields a metal-borane σ complex. Displacement of the weakly coordinated borane by P(OEt)3 proceeds by a dissociative mechanism and an Eyring analysis yields the first estimate of 20 kcal mol(-1) for the strength of this novel interaction.