Theoretical study on structures and aromaticities of a new series of sandwich complexes: [M2(η5P5)2] and [M(η5P5)2] (M = Be, Mg, and Ca)

Structures and magnetic properties [nucleus independent chemical shifts (NICS)] of [M2(η5-P5)2] and [M(η5-P5)2] (M = Be, Mg, and Ca) are studied with density functional theory at B3LYP level using the 6-311+G(d, p) basis sets. The M22+ and M2+ sandwich complexes with D5 symmetry are minima. The D5h and D5d symmetry conformations are saddle points on the flat potential energy surface. Analyses of molecular orbital correlation and binding energy for the two series of complexes reveal that the M-M bond is a weak σ covalent bond. In addition to electrostatic interactions, there are also covalent bonds between the M and the P5- ring. The M-M bond plays a dominant role in the stability of both series of complexes. The M-M and M-P5 bond strength decreases as M varies from Be to Ca. The P-P bond length in these complexes is slightly elongated with respect to that in P5-. The NICS computed with GIAO-B3LYP/6-311+G(d,p) indicates that the P5 rings in both series of complexes are aromatic, and the aromaticity decreases as M varies from Be to Ca. In these complexes, the NICS at the P5 ring center slightly decreases and it increases at the outer side of the P5 ring, thus resulting in the elongation of P-P bond and the significant flow of π-electron from the ring towards the bonding of the P5 ring with the M22+ unit or M and leading to the strengthening of the M-P5 bonding. The dissected NICS of the P5- ring of these complexes shows that the large total NICS is mainly due to the P-P π bond contribution of the P5 ring. The NICS of the P-P π bond decreases as the metal varies from Be to Ca.