Mutual Influences of Ligands as Revealed by the 2H NMR Chemical Shifts and the Angular Overlap Model Parameters: cis‐[CrX2(N)4] and fac‐[CrX3(N)3]‐Type Complexes with Aromatic Amines

Abstract

AbstractA mutual influence of ligands through a π donor–acceptor interaction is found by examining how the X co‐ligands of the aromatic amine complexes cis‐[CrX2([D4]bpy)2]n+ (bpy = 2,2′‐bipyridine) and fac‐[CrX3([D12]tpa)]n+ [tpa = tris(2‐pyridyl)amine] affect the 2H NMR chemical shifts in connection with the Angular Overlap Model (AOM) parameterization in an analogous manner to our previous study on the solvatochromism and solvent‐dependent 2H NMR spectra of cis‐[CrF2([D4]bpy)2]+ and fac‐[CrF3([D12]tpa)]. The ranking of the AOM parameter for monodentate and/or didentate ligands can be obtained from this co‐ligand effect only from the 2H NMR shifts. The postulated relative order in the two‐dimensional spectrochemical series is Cl– > Br– > O or N donors (mal2– > acac– or H2O > dmf > NCS–) for the aromatic amine complexes, which contrasts with the order O or N donors > Cl– > Br– for the aliphatic amine complexes(see the preceding paper in this issue). This fact suggests that the Cr–Cl and Cr–Br σ bonds are stronger in the soft acidic aromatic amine complexes than those in the hard acidic aliphatic amine complexes. This means they have a higher affinity for soft basic Cl– and Br– ligands and can be explained in terms of the hard/soft acid/base (HSAB) concept. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)