In this paper, at first, the possible structures for E5 n Zintl ions (E = Si, Ge, Sn, Pb; n = 0, 2, 4) were investigated at the BP86/def2-TZVPP and M06-2X/def2-TZVPP levels of theory. The results showed that E5 2 bare clusters, in agreement with their solid state structures, have the minimum energy in the trigonal bipyramid (D3h), while the neutral E5 and also E5 4 ions, which have not been characterized structurally to date, are stabilized in the oblate trigonal bipyramid (D3h) and square pyramid (C4v) geometries, respectively. Then [E5{M(CO)3}2]4 complexes (M = Cr, Mo, W) containing a planar E5 4 ring were investigated at the same levels of theory. The results show that the strong interaction of the planar E5 4 ring with the two {M(CO)3} fragments and significant charge donation from it is the reason for its strained planar geometry. This was further confirmed by NBO and Energy Decomposition Analysis- Natural Orbital for Chemical Valence (EDA-NOCV) as well as analysis of carbonyl vibrational frequencies. The EDA analysis indicated that the interaction energy values (DEint) in the series of these complexes show the same trend as the term for orbital interactions (DEorb).
