Interplay of Hückel and Möbius Aromaticity in Metal‐Metal Quintuple Bonded Complexes of Cr, Mo, and W with Amidinate Ligand: Ab initio DFT and Multireference Analysis**

Abstract

The aromaticity of metal‐metal quintuple bonded complexes of the type M2L2 (M=Cr, Mo, and W; L=amidinate) are studied employing gauge including magnetically induced ring current (GIMIC) analysis and electron density of delocalized bonds (EDDB). It is found that the complexes possess two types of aromaticity: i) Hückel aromaticity through delocalization of ligand π electrons with metal‐metal δ‐bond‐forming 6 conjugated electrons (4π and 2δ) ring; ii) Craig‐Möbius aromaticity through delocalization of π electrons of both the ligands with metal d‐orbitals in Craig type orientation forming 10π electrons ring with a double twist. Extended transition state natural orbital chemical valence (ETS‐NOCV) and canonical molecular orbital natural chemical shielding (CMO‐NCS) analysis confirm the Craig‐Möbius type arrangement of the orbitals. Furthermore, the unprecedented Hückel and Möbius type aromaticity is confirmed from the plot of the current pathways using 3D line integral convolution (3D‐LIC) plots. The metal‐metal bond order also increases down the group as justified from the complete active space self‐consistent field (CASSCF) analysis. Due to an increase in the π and δ electron conjugation, both the Hückel and Möbius aromaticity increase down the group.

CB6Al0/+: Planar hexacoordinate boron (phB) in the global minimum structure

“…Thus, to gain more insight into aromaticity/antiaromaticity/nonaromaticity the ring current calculations were performed using the GIMIC method. [74][75][76] The clockwise current (positive current) is called the diatropic current, and the anticlockwise current (negative current) is termed the paratropic current. The ring currents were calculated using the Gauss quadrature method in a cross-section area of a molecule.…”

Local reactivity descriptors of the important atoms in chelotropic reactions provide insight into their global variants along the reaction path

“…In general, for aromatic molecules, the clockwise (diatropic) current is present outside the ring, and the anticlockwise current (paratropic) is present inside the ring. In previous works [78–82], we have shown that it is a compelling technique for the analysis of aromaticity. Here also, we have employed the GIMIC method to calculate the ring current across various bonds at the TS structures of all four reactions (Table 5, Figure 8).…”

“…Möbius aromaticity arises when there is a phase shift in the molecule. This can be achieved in two ways [78]: (i) through nuclear arrangement (a twist in the molecule), and Heilbroner type; (ii) through orbital arrangement, Craig type. Here the molecules are showing Craig type Möbius aromaticity.…”

“…In previous works [78][79][80][81][82], we have shown that it is a compelling technique for the analysis of aromaticity. Here also, we have employed the GIMIC method to calculate the ring current across various bonds at the TS structures of all four reactions (Table 5, Figure 8).…”

“…[78]: (i) through nuclear arrangement (a twist in the molecule), and Heilbroner type; (ii) through orbital arrangement, Craig type. Here the molecules are showing Craig type Möbius aromaticity.…”