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

A 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 ‐[CrX 2 ([D 4 ]bpy) 2 ] n + (bpy = 2,2′‐bipyridine) and fac ‐[CrX 3 ([D 12 ]tpa)] n + [tpa = tris(2‐pyridyl)amine] affect the 2 H 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 2 H NMR spectra of cis‐ [CrF 2 ([D 4 ]bpy) 2 ] + and fac ‐[CrF 3 ([D 12 ]tpa)]. The ranking of the AOM parameter for monodentate and/or didentate ligands can be obtained from this co‐ligand effect only from the 2 H NMR shifts. The postulated relative order in the two‐dimensional spectrochemical series is Cl – > Br – > O or N donors (mal 2– > acac – or H 2 O > 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)