Electronic structure and bonding studies on triple-decker sandwich complexes with a P₆ middle ring

Abstract

DFT and hybrid HF-DFT studies of structure and bonding of CpMP₆MCp triple-decker sandwich complexes, ranging from 18–28 valence electrons (VE) with M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W, show that the middle P₆ ring complexes adopt symmetric planar (28 valence electron count [VEC]), asymmetric planar (26 VEC), and puckered (24 VEC) geometries. According to the mno Rule, 50 skeletal electrons are needed for these triple-decker cluster frameworks. For 28 VEC, this corresponds to 10 electrons more than the 50 electrons of the mno Rule if all VE of the metal are included. These additional electrons control the distortion of a P₆ middle ring and other finer structural details. Completely filled 2a^∗ and 2b^∗ orbitals in 28 VE complexes lead to a planar symmetrical P₆ middle ring, while the occupancy in either 2a^∗ or 2b^∗ alone explains the in-plane distortions (asymmetric) in 26 VE complexes. In comparison with 28 VE complexes, the puckering of P₆ middle ring in 24 VE complexes is due to the greater stabilization of 5a and the extra stabilization of the +4 oxidation state of Ti. The quintet state of 22 VE complexes is planar as 2a^∗ and 2b^∗ are half filled. Similar geometrical and bonding patterns of CpScP₆ScCp and C₂P₃H₂ScC₃P₃H₃ScC₂P₃H₂ support the carbon–phosphorus analogy further. The 18 VE systems, CpScC₃B₃H₆ScCp⁺ and CpScP₃B₃H₃ScCp⁺, have the 50 skeletal electrons as stipulated by the mno Rule. Corresponding anions have 52 skeletal electrons (20 VE); the middle rings here are distorted in the plane.