CHARACTERIZATIONS OF NOVEL BINUCLEAR ALKALINE-EARTH METAL COMPOUNDS: M2(ηn-N5)2 (M=Be AND Mg, n = 1, 2; M=Ca, n = 2, 5)

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Abstract
The first structural characterization for the twelve binuclear alkaline-earth metal compounds M2(ηn-N5)2 (M=Be, Mg; n = 1, 2) and Ca2(ηn-N5)2(n = 2, 5) have been optimized with local energy minimum by density functional theory (DFT). The most energetically favored structures in M2(ηn-N5)2(M=Be, Mg, Ca) are of D2d symmetry Be2(η1-N5)2, Mg2(η2-N5)2 and Ca2(η2-N5)2 and the metal–metal distances are 2.03 Å for Be-Be, 2.77 Å for Mg-Mg and 3.72 Å for Ca-Ca, which are significantly shorter than the experiment values of weakly bound bare diatomic Be2, Mg2 and Ca2.1,2Ca2(η5-N5)2 (D5d or D5h) is the only stable specie with sandwiched structure, bearing an even shorter Ca-Ca distance of 3.66 Å, and lying 24 kcal/mol higher in energy than the D2d structure. The dissociation enthalpies of the twelve M2(ηn-N5)2(M=Be, Mg, Ca) to two M(ηn-N5) fragments are predicted to be 72.6–73.1, 41.2–43.8, and 27.4–29.7 kcal/mol, respectively, implying a substantial metal–metal bonding. Natural bond orbital (NBO) analysis suggests that metal–metal bonds are of σ-bond. The natural charge of the alkali earth metal atom in the twelve M2(ηn-N5)2 species is larger than +0.88, which is consistent with the +1 oxidation state of the metal atoms. Nucleus-independent chemical shift (NICS) values confirm that the planar exhibits characteristics of aromaticity for these M2(ηn-N5)2 species.
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