Abstract
Our results from structural investigations of transition metal complex ion pairs in solution by homonuclear and heteronuclear 1D‐ and 2D‐NOE NMR spectroscopy are reviewed. Both model (A) and catalytically active (B) complexes have been taken into account; their general formulas are: trans‐[ML~2~(N,X)(Y)(Z)]^+^A^−^ (A) {M = Fe^II^, Ru^II^ and Os^II^, L = PMe~3~, Y = CO, Z = COMe, A^−^ = BPh~3~R^−^, CF~3~SO~3~^−^, BF~4~^−^, PF~6~^−^ and B[3,5‐(CF~3~)~2~C~6~H~3~]~4~^−^; M = Ir^III^, L = PR~3~, Y = Z = H and A^−^ = BPh~4~^−^, CF~3~SO~3~^−^, BF~4~^−^, PF~6~^−^} and [M′(N,X)(R,olefin)]^+^A^−^ (B) {M′ = Pt, R = Me, olefin = ethene, propene and CH~2~=CHCOOMe, A^−^ = BF~4~^−^; M′ = Pd, (R,olefin) = η^1^,η^2^‐5‐methoxycyclooctenyl, A^−^ = BPh~4~^−^, CF~3~SO~3~^−^, BF~4~^−^, PF~6~^−^, SbF~6~^−^ and B[3,5‐(CF~3~)~2~C~6~H~3~]~4~^−^}, where N,X = bis(pyrazolyl)methane, bipyridine, α‐diimines, α‐diamines or α‐oxo imines. The detection of dipolar interionic interactions in ^1^H‐NOESY and ^19^F,^1^H‐HOESY NMR spectra allowed relative anion‐cation orientations to be determined, these being well defined in most cases, with one orientation predominating in solution. We often found that the anion approaches the complexes from the side of the N,X‐ligands, due to an accumulation of positive charge at the junction of the moieties containing the N‐ and X‐donor atoms. In square‐planar complexes B, the apparently favoured apical positions were often protected either by electronic factors [N,X = bis(2‐pyridyl)amine or bis(pyrazolyl)methane] or by a combination of electronic and steric factors (N,X = α‐diimines). For M′ complexes, bearing Ar−N=C(Me)−C(Me)=N−Ar ligands, the position of the counter‐anion is finely modulated by the steric hindrance of the ortho‐aryl substituents. The accessibility of the metal centre, which is directly associated with the catalytic performances of the complexes, was determined by using the counteranion as a probe. Finally, average interionic distances were estimated for model complexes A, bearing unsymmetrical BPh~3~R^−^ counter‐anions, based on the quantification of interionic NOEs. (© Wiley‐VCH Verlag GmbH, 69451 Weinheim, Germany, 2003)