Despite B and Al belong to the same group 13 elements, the B6²- cluster prefers the planar D2h geometry, whereas Al6²- favours the Oh structure. In this work, we analyse the origin of the relative stability of the D2h and Oh forms in these clusters by means of an energy decomposition analysis based on the turn-upside-down approach. Our results show that what causes the different trend observed is the orbital interaction term, which combined with the electrostatic component do (Al6²- and Ga6²-) or do not (B6²-) compensate the higher Pauli repulsion of the Oh form. Analysing the orbital interaction term in more detail, we find that the preference of B6²- for the planar D2h form has to be attributed to two particular molecular orbital interactions. Our results are in line with a dominant delocalisation force in Al clusters and the preference for more localised bonding in B metal clusters. For mixed clusters, we have found that those with more than two B atoms prefer the planar structure for the same reasons as for B6²-

This work has been supported by the Ministerio de Economía y Competitividad (MINECO) of Spain (Project CTQ2014-54306-P) and the Generalitat de Catalunya (project 2014SGR931, Xarxa de Referència en Química Teòrica i Computacional, ICREA Academia 2014 prize for M.S., and grant No. 2014FI_B 00429 to O.E.B.). The EU under the FEDER grant UNGI10-4E-801 (European Fund for Regional Development)has also funded this research. J. P. thanks the National Research School Combination - Catalysis (NRSCC), and The Netherlands Organization for Scientific Research (NWO/CW and NWO/NCF)