SOLUTION STRUCTURE AND DYNAMICS OF 5-COORDINATE NICKEL(II) COMPLEXES WITH TRIMETHYLPHOSPHINE - NIX2L3, [NIXL4]+, [NIL5]2+

The following complexes have been characterized in CH2C12 or CHC1F2 solutions between 114 and 295 K by 31P(1H) FT NMR and electronic spectroscopies: NiX2L3 (X=CN, Cl, Br, or I), [NiXL4]X (X=Cl, Br, or I), and [NiL5]2+ (with L=PMe3). These complexes present a trigonal-bipyramidal geometry with the halide(s) in equatorial position and the cyanides in axial position. The intramolecular PMe3 exchange in [NiL5](BF4)2 (ΔG*=6.2 kcal mol-1 at 130 K) and in [NiXL4]BF4 (X=Cl (6.6), Br (7.8), I (8.2) at 169 K) proceeds via a Berry pseudorotation. For [NiXL4]X, the exchange rate is faster, proportional to the concentration of the ionic halide, and increases according to Cl<Br <I; an la mechanism with a 20-electron transition state is suggested. In NiX2(PMe3)3 the energy barriers for the axial-equatorial phosphine exchange are higher (9.2-13.0 at 230 K); a D mechanism, with loss of an axial phosphine to form a four-coordinate intermediate, is proposed. The intermolecular exchange reactions [FORMLA OMMITED] are dissociative D. © 1979, American Chemical Society. All rights reserved.