On the Correlation of 35Cl Nuclear Quadrupole Coupling Constants with π→γ3 and π→γ5 Optical Electron Transfer Bands of Transition Metal Complexes and its Significance in Pi-Bonding

Abstract

From a consideration of odd-even intramolecular electron transfer processes in octahedrally co-ordinated transition metal complexes, two relationships have been derived. The first correlation describes π → γ3 optical electron transfer bands of the complexes K2OsCl6, K2IrCl6, K2PtCl6, K2PtCl4 as a function of the respective 35Cl nuclear quadrupole coupling constants. The second relationship correlates π → γ5 optical charge transfer transitions of the substances K2ReCl6, K2OsCl6, K2IrCl6 with the 35Cl nuclear quadrupole coupling constants of these compounds. By using this charge transfer model, a linear relationship has been verified between the optical electronegativity values of the quadrivalent transition metal ions W4⨁, Re4⊖, Os4⨁, Ir4⨁, Pt4⨁ and the 35Cl nuclear quadrupole coupling constants of their hexachloro complexes. The empirically established equation χMen+ = 0.537 which describes the 35Cl n.q.r. frequencies of the hexachloro complexes of Re4⨁, Os4⨁, Ir4⨁, Pt4⨁ and Pd4⨁ as a function of the optical electronegativity values χ of these metal ions, has been used to estimate 35Cl nuclear quadrupole resonance frequencies of chloro complexes as yet not probed by nuclear quadrupole resonance spectroscopy, e.g. Zeise's salt (KPtCl3C2H4), K3IrCl6, K2RuCl6, K3RhCl6 and K3MoCl6. Conversely, by the same procedure the optical electronegativity χ of W4⨁ and Au3⨁ has been estimated from the 35Cl quadrupole resonance frequency of K2WCl6 and KAuCl4 (χ W4⨁= 1.71 and χ Au3⨁=2.80). In the case of analogous bromo complexes, the equation χ Men+ = 0.19 mol accounts for the dependence of the 79Br nuclear quadrupole resonance frequencies upon the optical electronegativity of the metal ions. The assignment of electron transfer bands in other chloro complexes is discussed in the light of this “nuclear quadrupole resonance criterion“ of optical charge transfer bands.