Abstract: The equilibrium geometries, binding energies and aromaticities of (1,3,5-C₃P₃H₃)M and (1,3, 5-C₃P₃H₃)₂M (M=Ti, V, Cr) were calculated by density function theory. The results indicate that the ground states of (1,3,5-C₃P₃H₃)M and (1,3,5-C₃P₃H₃)₂M have C_3v and D_3h symmetries, respectively. The main interactions between the ligands and metal are covalent interactions featuring three types of interactions represented as σ, π and δ between the ligands and the metal. The dissociation method of the ligands and the metal in sandwich V complexes is different from that of Ti and Cr complexes, i.e., the former consists of two steps and the latter consists of one step. The first dissociation energy of (1,3,5-C₃P₃H₃)₂Cr is the largest and so it is the most stable one. These complexes have central, inner and outer aromaticities and the central-aromaticities of the complexes are stronger than that of (1,3,5-C₃P₃H₃). The contributions of aromaticities is dominated by π bonds and the lone pair electronics of the metal atom. The inner-aromaticities of the complexes increase in the following order: Ti, V, Cr, and they are evidently stronger than the outer-aromaticities. Compared with (1,3,5-C₃P₃H₃)Ti (C_3v, ¹A₁) the distortion of the ligands for the high spin multiplicity of half-sandwich (1,3,5-C₃P₃H₃)Ti (C₃, ⁵A₁) is larger and more stable. The central and inner aromaticities in the C plane of the high spin multiplicity half-sandwich (1,3,5-C₃P₃H₃)Ti (C₃, ⁵A₁) are stronger than that of (1,3,5-C₃P₃H₃)Ti (C_3v, ¹A₁), but the central aromaticity in the P plane is weaker.

Key words: Density functional theory, Sandwich complex, Structure, Aromaticity