Geometries, stability and aromaticity of Al₂ P₂^{2 -}, [M(Al₂P₂)]^- (M = Li, Na, K, Cu) and N(Al₂P₂) (N = Be, Mg, Ca, Zn) clusters

Geometrical structure, aromaticity and other properties of Al₂ P₂^{2 -}, [M(Al₂P₂)]^- (M = Li, Na, K, Cu) and N(Al₂P₂) (N = Be, Mg, Ca, Zn) species are theoretically investigated with density functional theory (DFT) methods. Calculation results show that for Al₂ P₂^{2 -} species, the planar structure, with D_2h symmetry at the ¹A_g state, is the global minimum at the B3LYP/6-311+G_* level. Natural bond orbital (NBO) analysis indicates the existence of delocalization in the most stable Al₂ P₂^{2 -} species and its pyramidal complexes. Nucleus-independent chemical shift (NICS) and molecular orbital (MO) analysis further reveal that that pyramidal [M(Al₂P₂)]^- and N(Al₂P₂) species preserve the aromatic nature of the most stable Al₂ P₂^{2 -} unit.