Theoretical study on structures and aromaticities of a new series of sandwich complexes: [M 2(η 5[sbnd]P 5) 2] and [M(η 5[sbnd]P 5) 2] (M = Be, Mg, and Ca)

Abstract

Structures and magnetic properties [nucleus independent chemical shifts (NICS)] of [M 2(η 5–P 5) 2] and [M(η 5–P 5) 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 M 2 2 + and M 2+ sandwich complexes with D 5 symmetry are minima. The D 5 h and D 5 d 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 P 5 - ring. The M–M bond plays a dominant role in the stability of both series of complexes. The M–M and M–P 5 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 P 5 - . The NICS computed with GIAO-B3LYP/6-311+G(d,p) indicates that the P 5 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 P 5 ring center slightly decreases and it increases at the outer side of the P 5 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 P 5 ring with the M 2 2 + unit or M and leading to the strengthening of the M–P 5 bonding. The dissected NICS of the P 5 - ring of these complexes shows that the large total NICS is mainly due to the P–P π bond contribution of the P 5 ring. The NICS of the P–P π bond decreases as the metal varies from Be to Ca.