STRETCHED H2 COMPLEXES - STRUCTURAL CONTROL IN REH7(P(C6H4-P-X)3)2 (X = CH3, H, F, CF3, OME) COMPLEXES BY ELECTRONIC LIGAND EFFECTS

A series of new or known ReH7{P(C6H4-p-X)3}2 (X = CH3 (1), H (2), F (3), CF3 (4), OMe (5)) complexes which contain isosteric phosphine ligands are studied by H-1 NMR spin-lattice (T1) hydride relaxation time measurements. The T1(min) values are found to decrease as the electron-withdrawing ability of the aromatic para substituents X of the phosphines increases. A plot of log {T1X(min)/T1H(min)} against the Hammett sigma(p) constants for the substituents gives a rho-value of -0.30. The results are consistent with the formulation of 1-5 in solution as containing an elongated eta-2-H-2 ligand with different H...H distances (for compound 1, r(H...H) = 1.357 angstrom by neutron diffraction) which depend primarily on the electronic character of the phosphine ligands and increase from 1.24 angstrom for the most electron-withdrawing phosphine P(C6H4-P-CF3)3 to 1.42 angstrom for the most electron-donating phosphine P(C6H4-P-OMe)3.