Measurement of magnitude and sign of heteronuclear coupling constants in transition metal complexes

Sets of specifically tailored E.COSY-type correlation experiments and doubl e-quantum/zero-quantum (DQ/ZQ) experiments are presented which enable the d etermination of sign and size of small heteronuclear coupling constants acr oss the metal center of transition metal complexes. For the octahedrally co ordinated complexes, [Ru(TPM)(H)(CO)(PPh3)](+)[BF4](-) (1) and [Ir(TPM)(H)( CO)(CO2CH3)](+)[BF4](-) (2), 14 of 15 and 15 of 15 possible two-bond scalar coupling constants across the metal center were measured, respectively, us ing N-15 and N-15/C-13 enriched samples (TPM = tris(1-pyrazolyl)methane)). The reduced coupling constants K-2(X-M-Y) = 4 pi(2) (2)J/(h gamma(x)gamma(y )) were found to be positive when the coupled nuclei X and Y were trans wit h respect to the metal center, and negative when the coupled nuclei were in cis position. The validity of this sign rule was verified for J(CC), J(NN) , J(PN), J(PC), J(CN), J(HP), J(HC), and J(HN) couplings. Idiosyncracies as sociated with 2D NMR spectra for the sign determination of coupling constan ts with N-15 which lead to corrections for the signs of J(HN), J(PN), and J (CN) couplings reported previously are discussed. (C) 1999 Academic Press.