On predicting bonding patterns of small clusters of alkaline-earth (Be, Mg) and triel (B, Al) fluorides: a balance between atomic size and electron-deficient character

The structures, bonding and stability of (MF2)(m):(M ' F-3)(n) (M = Be, Mg; M ' = B, Al; m = 0,1,2; n = 0,1,2) clusters were obtained at the B3LYP/aug-cc-pVTZ level of theory. To understand trends across this set of closely related atoms, an analysis of the results obtained using atoms in molecules (AIM), electron localisation function (ELF) and electron density shift (EDS) approaches, permits to identify subtle dissimilarities when the first-row elements, Be and B, are replaced by the second-row counterparts, Mg and Al. For dimers this replacement involves an increase in the bonding enthalpies as a direct consequence of a much larger ionic character of the derivatives including second-row elements. For trimers and tetramers, rather stable structures involving penta-coordinated aluminium atoms are formed, which are not found for the B-containing analogues. In all clusters investigated the electronic environment around each monomer does not change significantly neither with nature of the monomers interacting with it or with the size of the cluster, though some small cooperative effects are observed when analyzing the binding enthalpies. The important consequence is that the stability of larger clusters can be easily predicted through a statistical treatment of the values obtained for the smaller ones.