CROWDED IRON HYDRIDE COMPLEXES - X-RAY CRYSTAL-STRUCTURES OF [FE(C5H5)(DPPM)(H)] AND OF [FE(C5H5)(PPH3)2(H)], AND THEIR CORRELATION WITH THE DISTINCTION OF THE METHYLENE PROTON IN THE DPPM LIGAND BY NMR

The X-ray crystal structures of two crowded complexes [Fe(II)Cp(dppm)(H)], 2a, and [FeIICp(PPh3)2(H)], 2b are reported. These complexes were obtained by low-temperature reaction of the phosphine with the 19-electron complexes [FeICP(C6H5R)], R = H or Me, 1. The location and orientation of the phosphine phenyl rings with respect to the two rigidly-held dppm methylene protons of 2a, and the 4J(HH) coupling of these protons with Fe-H, allows their assignment in the NMR spectrum of 2a. 2a. [Fe(C5H5)(dppm)(H)]; C30H28P2Fe(H); M(x) = 506.35, monoclinic, P2(1)/n, a = 16.826 (3), b = 9.822 (1), c = 17.288 (3) angstrom and beta = 118.26 (1)degrees, Z = 4, lambdaCuKalpha = 1.54182 angstrom, R = 0.033 for 2 492 observed reflections. 2b. [Fe(C5H5)(PPh3)2(H)] tetrahydrofuran solvate; C41H35P2Fe(H), C4H8O; M(x) = 734.6 triclinic, P1BAR, a = 10.388 (1), b = 10.808 (1), c = 16.788 (2) angstrom, alpha = 90.42 (1), beta = 96.28 (1) and gamma = 99.61 (1)degrees, Z = 2, lambdaCuKalpha = 1.54182 angstrom, R = 0.043 for 5 888 observed reflections. The location and orientation of the two C-H bonds in the dppm CH2 moiety, those of the Fe-H bond, and the remarkable difference in the coupling constant 4J(HH) between Fe-H and each of the two CH2 protons allow the assignment of the H-1 NMR signals. The large 4J(HH) value is attribuable to the "M" configuration (figure 4, H(b)H(c)).