N4 ring as a square planar ligand in novel MN4 species

Ab initio (MP2) and density functional theory (DFT) methods are used to examine a series of MN4 compounds, where M is an alkaline-earth cation (Mg2+, Ca2+, Sr2+, Ba2+), and N4(2-) is a six--electron ring. All pyramidal structures except MgN4 are the most energetically favored for all singlet MN4 systems considered here. For MgN4, the CS structure with dicoordinated Mg out of the N4 ring plane is the most stable of all. Among these systems, the pyramidal CaN4, SrN4, BaN4 and the planar C(S) structure containing dicoordinated Ba are stable as singlet molecules due to their significant isomerization or dissociation barriers (21.3-94.1 kcal/mol). Structural, natural bond orbital (NBO), and molecular orbital (MO) analyses indicate that the bonding in the BaN4 system has a larger covalent character as compared with other MN4 systems. In addition, substantial d character is found in the bonding of the MN4 (M = Ca2+, Sr2+, Ba2+) species.