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

Abstract

The first structural characterization for the twelve binuclear alkaline-earth metal compounds M 2(ηn- N 5)2 ( M=Be , Mg ; n = 1, 2) and Ca 2(ηn- N 5)2(n = 2, 5) have been optimized with local energy minimum by density functional theory (DFT). The most energetically favored structures in M 2(ηn- N 5)2( M=Be , Mg , Ca ) are of D2d symmetry Be 2(η1- N 5)2, Mg 2(η2- N 5)2 and Ca 2(η2- N 5)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 Be 2, Mg 2 and Ca 2.1,2 Ca 2(η5- N 5)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 M 2(ηn- N 5)2( M=Be , Mg , Ca ) to two M (ηn- N 5) 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 M 2(ηn- N 5)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 [Formula: see text] exhibits characteristics of aromaticity for these M 2(ηn- N 5)2 species.