The reactions of Co-2(CO)(8) with the ferraboranes B2H6Fe2(CO)(6), HFe3(CO)(9)BH4, HFe3(CO)(10)BH2, and HFe4(CO)(12)BH2 lead to fragment addition and/or fragment exchange. Thus, the new mixed-metal metallaboranes B2H5FeCo(CO)(6) (1), Fe2Co(CO)(9)(mu-CO)BH2 (2), FeCo2(CO)(9)(mu-CO)BH (3), FeCo2(CO)(9)(BH)(2) (4), and HFe3Co(CO)(12)BH (5) have been isolated in modest yields and characterized spectroscopically. Comparison of selected spectroscopic and chemical properties of isoelectronic pairs of hetero- and homometallaboranes defines the perturbation of the cluster network by the replacement of HFe by Co. Both direct effects due to the different metal atoms and indirect effects resulting from the differing number of skeletal hydrogen atoms are observed. Consistent with the radical mechanism suggested for Co(CO), fragment addition to and exchange with organometallic clusters, a radical mechanism is proposed. However, the metallaborane reactions require neutral molecules and appear to be initiated by H atom transfer whereas the analogous anionic organometallic reactions proceed via electron transfer.