Measurement of heteronuclear coupling constants in organometallic complexes using high-resolution 2D NMR

¹H-³¹P coupling constants across the central metal atom (²J_P-M-H) were measured in a series of octahedral hydridometalphosphine complexes [MRH(PP₃), PP₃ = P(CH₂CH₂CH₂P(CH₃)₂) ₃; M = Ru, Fe; R = H, Cl, 1-pentenyl, cyclopentenyl, 2-methyl-1-butenyl, 3,3-dimethyl-1-butenyl] using a selective two-dimensional NMR experiment. The use of band-selective pulses focused on a narrow frequency range in the ¹H NMR spectrum (encompassing the metal-bound hydride resonances) provided spectra with high resolution as well as removed the need for suppression of intense signals arising from protic organic solvents. The magnitude of ²J_P-M-H was found to depend on the relative disposition of the coupled nuclei and about the central metal atom, as well as the nature of the metal. For FeRH(PP₃) complexes, the magnitude of ²J_P-M-H(cis) > ²J_P-M-H(trans), while for analogous Ru complexes, ²J_P-M-H(trans)>²J_P-M-H(cis). Irrespective of whether the metal center is Fe or Ru, the sign of ²J_P-M-H(trans) is opposite that of ²J_P-M-P(cis), and ²J_P-M-P(cis), but the same as ²J_P-M-P(trans).