CHARACTERIZATIONS OF NOVEL BINUCLEAR ALKALINE-EARTH METAL COMPOUNDS: <font>M</font>₂(ηⁿ-<font>N</font>₅)₂ (<font>M=Be</font> AND <font>Mg</font>, n = 1, 2; <font>M=Ca</font>, n = 2, 5)

Abstract

The first structural characterization for the twelve binuclear alkaline-earth metal compounds M 2 (η n - N 5 ) 2 ( M=Be , Mg ; n = 1, 2) and Ca 2 (η n - 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 (η n - N 5 ) 2 ( M=Be , Mg , Ca ) are of D 2d symmetry Be 2 (η 1 - N 5 ) 2 , Mg 2 (η 2 - N 5 ) 2 and Ca 2 (η 2 - 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 (η 5 - N 5 ) 2 (D 5d or D 5h ) 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 D 2d structure. The dissociation enthalpies of the twelve M 2 (η n - N 5 ) 2 ( M=Be , Mg , Ca ) to two M (η n - 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 (η n - 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 (η n - N 5 ) 2 species.