A comprehensive study has been carried out to investigate the metathesis reactivity of the terminal alkylborylene complex [(eta(5)-C5H5)(OC)(2)Mn=B(tBu)] (1). Its reactions with 3,3',5,5'-tetrakis(trifluoromethyl)benzophenone, 4,4'-dimethylbenzophenone, 2-adamantanone, 4,4'-bis(diethylamino)benzophenone, and 1,2-diphenylcyclopropen-3-one afforded the metathesis products [(eta(5)-C5H(5))(OC)(2)Mn=CR2] (R = C6H(3)-3,5-(CF3)(2) 3a, C6H4-4-Me 3b, C6H4-4-NEt2 3d; CR2 = adamantylidene 3c, cyclo-C3Ph2 3e). The cycloaddition intermediates were detected by NMR spectroscopy from reactions involving ketones with more electron-withdrawing substituents. The reaction of 1 with dicyclohexylcarbodiimide (DCC) only proceeds to form the cycloaddition product [(eta(5)-C5H5)(OC)(2)Mn{kappa(2)-C,B-C(=NCy)N(Cy)B(tBu)}] (4), which upon warming, rearranges to afford complex [(eta(5)-C5H5)(OC)(2)Mn{CN(Cy)B(tBu)CN(Cy)}] (5). The reaction of 1 with triphenylphosphine sulfide SPPh3 also yields the metathesis product [(eta(5)-C5H5)(OC)(2)Mn(PPh3)] via an intermediate which is likely to be a eta(2)-thioboryl complex [(eta(5)-C5H5)(OC)(2)Mn{eta(2)-SB(tBu)}] (6). Similar reactions have been studied using an iron borylene complex [(Me3P)(OC)(3)Fe=B(Dur)] (Dur = 2,3,5,6-tetramethylphenyl, 9). Extensive computational studies have been also carried out to gain mechanistic insights in these reactions, which provided reaction pathways that fit well with the experimental data.