Mutual Influences of Ligands as Revealed by 2H NMR Shifts and the Angular Overlap Model Parameters: trans‐ and cis‐[CrX2(N)4]‐Type Complexes with Aliphatic Amine Ligands

Abstract

AbstractWe have previously reported the relationship between 2H NMR chemical shift differences (Δδ) and the angular overlap model (AOM) parameter eσ(F), which represents the σ antibonding interaction energy obtained from the solvatochromism, for trans‐[CrF2([D4]‐3,2,3‐tet)]n+ {[D4]3,2,3‐tet = 1,10‐diamino‐4,7‐diazadecane [NH2(CH2)3NH(CD2)2NH(CH2)3NH2]} and cis‐α‐[CrF2([D4]trien)]n+ {[D4]trien = triethylenetetraamine [NH2(CH2)2NH(CD2)2NH(CH2)2NH2]}. In analogy to these relations, the co‐ligand effect on the Δδ values for cis‐α‐[CrX2([D4]trien)]n+ and trans‐[CrX2([D4]3,2,3‐tet)]n+ is found to be well correlated with the reported eσ(X) values for the co‐ligands X. This is the first example of the co‐ligand effect, or a mutual influence of ligands, associated with variation of the Cr–N and Cr–X bonds in cis‐ and trans‐[CrX2N4]‐type complexes. This Δδ vs. eσ(X) correlation offers an indirect method for estimating the eσ(X) values for didentate X2 as well as monodentate X ligands from the 2H NMR chemical shifts and ranking them in the two‐dimensional spectrochemical series. Plots of Δδ vs. eσ(FX) for trans‐[CrF(X)([D4]3,2,3‐tet)]n+ do not necessarily lie on the Δδ vs. eσ(X) correlation line for the F(X) complexes with averaged eσ(FX) = 1/2[eσ(X) + eσ(F)] values ― eσ[F(CN)] is close to eσ(F), for example ― thereby demonstrating a mutual influence between the CN– and F– ligands in the trans positions through a push‐pull action or π donor–acceptor interaction.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)