Clustering of Electron Deficient B‐ and Be‐Containing Analogues: In the Fight for Tetracoordination, Beryllium Takes the Lead

Despite being neighbors in the periodic table and highly electron‐deficient elements, beryllium and boron can lead to complexes with distinctive peculiarities. CCSD(T) calculations using 6‐31+G(d,p) and aug‐cc‐pVTZ basis sets on B3LYP optimized geometries with the same basis set expansions have been carried out to characterize the structures, bonding and stability of (BeX 2 ) m : (BX 3 ) n (n=0,1,2; m=0,1,2; X=F, Cl, CN, NC, CCH) complexes. The differences between Be and B are already manifested in their dimerization patterns, since (BeX 2 ) 2 homodimers are very stable, BeX 2 : BX 3 heterodimers are systematically less stable than Be‐homodimers, and (BX 3 ) 2 homodimers are in almost all cases not prone to form. This trend is a reflection of the strongly electropositive nature of Be compared with B. Larger clusters exhibit the same behavior, as no stable BeX 2 (BX 3 ) 2 trimers can be found with two BX 3 monomers being neighbors. Conversely, all (BeX 2 ) 2 (BX 3 ) trimers present, as a global minimum, a structure in which the two BeX 2 monomers are bonded, whereas for the (BeX 2 ) 2 (BX 3 ) 2 tetramers the (BeX 2 ) 2 subunit is at the center of the cluster. The relevance of this binding pattern in the global stability is supported by an excellent correlation between the stabilization energies and the number of Be−Be and Be−B interactions the clusters present, with a clear preference for maximizing facing Be subunits. One obvious consequence is that the stability of the complexes increases dramatically with their size in a fashion that can be easily predicted.