2,2'6',2''-TERPYRIDINE (TERPY) ACTING AS A FLUXIONAL BIDENTATE LIGAND .4. CIS-[M(C6F5)(2)(TERPY)] (M=PD OR PT) - NUCLEAR-MAGNETIC-RESONANCE STUDIES OF THEIR SOLUTION DYNAMICS AND CRYSTAL-STRUCTURE OF CIS-[PD(C6F5)(2)(TERPY)]

2,2': 6',2''-Terpyridine reacted with trans-[M(C6F5)(2)(diox)(2)] (M = Pd or Pt, diox = 1,4-dioxane) to form the square-planar complexes cis-[M(C6F5)(2)(terpy)] in which the terpyridine acts as a bidentate chelate ligand. in solution these complexes are fluxional with the terpyridine oscillating between equivalent bidentate modes by a mechanism consisting of a 'tick-tock' twist of the metal moiety through an angle equal to the N-M-N angle of the metal centre. Rates of this fluxion were measured by NMR spectroscopy from the exchange effects on the H-1 signals of the aromatic hydrogens and in the F-19 signals of two C6F5 groups. The Delta G(double dagger) values for the fluxion were ca. 71 and 94 kJ mol(-1) for the complexes of Pd-II and Pt-II respectively. At below-ambient temperatures further changes in the F-19 NMR spectra of both complexes were interpreted in terms of varying rates of rotation of the unco-ordinated pyridine ring, with the rates of rotation of the C6F5 rings being substantially slower at all temperatures and not separately measurable. The lowest-temperature spectra suggested the presence of a pair of degenerate rotamers each having the planes of both C6F5 rings and the unco-ordinated pyridine ring closely parallel and orthogonal to the remainder of the ligand ring system. The crystal structure of [Pd(C6F5)(2)(terpy)] confirms the bidentate chelate bonding of terpy with a N-Pd-N angle of 77.9 degrees, and the pendant ring oriented at an angle of 46 degrees to the adjacent co-ordinated ring.