A theoretical investigation of the electronic and molecular structure of polyenes coordinated to transition metal fragments

Abstract

Nonempirical, approximate, Fenske-Hall molecular orbital (FHMO) calculations were used to determine the complex and component fragment wavefunctions for the series of 18-electron, polyene-transition metal complexes, (C(,n)H(,n))ML(,m) {n=3-8; M=Ti-Ni; L=CO, CN('-), NO('+); m=2-3} in idealized, regular geometries for the staggered and eclipsed conformations. Total electron density distributions were computed from the molecular and fragment wavefunctions, and the fragment sum was subtracted from the total to yield deformation density ((DELTA)(rho)) maps of the molecule. The maps revealed electron density loss from carbon-carbon (C-C) bonds or carbon atoms in the complexed polyene which were eclipsed by a counter-ligand (L) and electron density gain by C-C bonds or carbon atoms which were not eclipsed by L. Thus, eclipsed C-C bonds are longer than uneclipsed bonds, and eclipsed carbon atoms are closer to the metal and are more positively charged than uneclipsed atoms. It is suggested that the charge differences in the eclipsed conformers lead to enhanced nucleophilic attack at the eclipsed carbons and enhanced electrophilic attack at the uneclipsed carbons. Ab initio self-consistent-field (SCF) MO calculations beyond the Hartree-Fock-Roothaan (HRF) single determinant level were performed on ((eta)('4)-C(,4)H(,4))Fe(CO)(,3) using the generalized-molecular-orbital (GMO) technique with configuration interaction (CI) to correctly assign the ultraviolet photoelectron spectrum of the complex. The upper valence bands, which are predicted to be in the wrong order by Koopmans' Theorem and (DELTA)SCF calculations, are correctly ordered under the GMO-CI procedure, although the band gap is large. The correlated GMO-CI wavefunction is shown to correctly describe the unusual electrophilic reactivity of complexed cyclobutadiene. ...