A systematic ab initio molecular orbital (MO) study of nearly all possible degenerate cluster rearrangement mechanisms for B6H10 has been performed at the MP2/6-31G* and the HF/6-31G levels. All the feasible mechanisms have been found to involve an initial diamond-square-diamond (DSD) rearrangement with a high activation energy to form the B5H8(BH2) tetragonal pyramidal intermediate having a bridging BH2 group. A path from B5H8(BH2) through the middle structure of C-2 symmetry has been shown to be the most favorable for the apical-basal rearrangement. The preferable mechanism for basal-basal reorganization involves scrambling of the bridging BH2 group from one basal edge to another in B5H8(BH2) and proceeds via the middle structure of nido-type with one square open face. For both mechanisms the initial DSD rearrangement is the rate determining step with the barrier of 47 kcal/mol at the MP2/ 6-31G*+ZPE level. Any rearrangement mechanism with low activation energy is unlikely to exist. For [(IrB5H8)-(CO)(PH3)(2)] the isomerization process rearranging the Ir atom from a basal to the apical position is shown to occur by a mechanism similar to that of apical-basal rearrangement of B6H10. The calculated barrier, 39 kcal/mol at the MP2//HF/ECP-DZ level, is high enough to prevent the isomerization within reasonable temperature.