Nonclassical vs classical metal...H3C-C interactions: accurate characterization of a 14-electron ruthenium(II) system...

A neutron diffraction study of the complex RuCl(2)[PPh(2)(2,6-Me(2)C(6)H(3))](2) (1) defines the precise nature of the delta agostic interactions between the unsaturated metal center and two o-methyl groups of the xylyl substituents. The CH(3) carbon atoms lie in the RuP(2) equatorial plane with Ru...C distances of 2.637(7) and 2.668(6) A, whereas four short Ru.H distances (from 2.113(11) to 2.507(11) A) indicate that each methyl group interacts with two C-H bonds. A survey of the X-ray structures with beta, gamma, delta, and epsilon M...H(3)C-C moieties (no neutron data have been previously reported) shows a linear correlation between the angle M.C-C and the torsion of the methyl group about the C-C bond. Thus, the agostic interactions span the range between the classical (M...eta(2)-HC) and the nonclassical (M...eta(3)-H(2)C) types. A solution study of 1 shows intramolecular rearrangement of each xylyl substituent that equilibrates the environments of its two ortho CH(3) groups. Activation parameters, evaluated from the analysis of (1)H NMR line shape as a function of temperature, are Delta H(++) = 9.6 +/- 0.2 kcal mol(-1) with Delta S(++) = -15.4 +/- 0.7 eu (CDCl(3)). The related 14-electron complexes RuX(2)[PPh(2)(2,6-Me(2)C(6)H(3))](2) (X = I, 2; NCO, 3), prepared from 1 and NaX, show a similar dynamic process in solution, with the iodo derivative displaying the most hindered rotation of the xylyl group. A DFT optimization of the complex RuCl(2)[PH(2)(2,6-Me(2)C(6)H(3))](2) (1a) reproduces well the nonclassical Ru...eta(3)-H(2)C agostic mode, whereas the classical Ru...eta(2)-HC one corresponds to a transition state 1b, destabilized by 3.4 kcal mol(-1). A similar barrier (ca. 3.8 kcal mol(-1)) is calculated for the xylyl rotation in the further simplified model RuCl(2)[PH(2)(2,6-Me(2)C(6)H(3))][PH(2)CH[double bond]CHCH(3)] (1c), the absence of bulky phenyl substituents being largely responsible for the difference with respect to the experimental value. Finally, the MO analysis addresses the intrinsic stability of the 14-electron complex RuCl(2)(PH(3))(2) and, in agostic complexes, accounts for the different interactions between the methyl group and the metal atom in relation to the length of their interconnecting chain.