REACTION OF H-2 WITH IRHCL2P2 (P = P(I)PR(3) OR P(T)BU2PH) - STEREOELECTRONIC CONTROL OF THE STABILITY OF MOLECULAR H-2 TRANSITION-METAL COMPLEXES

IrHCl2P2 (P = PiPr3) reacts rapidly with H-2 at 25-degrees-C to set up an equilibrium where H-2 binds trans to the original hydride ligand ( trans-2). A second slower reaction forms IrH(H2)Cl2P2 (cis-2), where t he cis disposition of the chlorides, and also H cis to H-2, was establ ished by neutron diffraction. This molecule (unlike trans-2), shows ra pid site exchange between coordinated H and H-2. cis-2 can be induced to lose HCl to form Ir(H)2ClP2 (3). The structure of Ir(H)2Cl(PtBu2Ph) 2, an analog of 3, was shown by neutron diffraction to have a planar H 2IrCl in a Y shape, with Cl at the base of the Y and a H-Ir-H angle of only 73-degrees. ECP ab initio calculations of IrH2Cl(PH3)2 show that the Y shape with a H-Ir-H angle close to the experimental value has t he minimum energy. They also show that the trans-2 isomer of IrH(H2)Cl 2(PH3)2 is less stable than the cis-2 isomer by 10.3 kcal/mol. The Ir- H-2 interaction is stronger in cis-2. The rotational barrier has been calculated in the two isomers as 2.3 (trans) and 6.5 (cis) kcal/mol. I n agreement with the experimental structure, the H-H bond is found to eclipse preferentially the Ir-H bond in cis-2. The calculations also s how that the Ir-H-2 bond dissociation energy is greater in cis-2. It t hus appears that the binding ability of a metal fragment not only depe nds on its ligands but is also linked in a subtle way to its stereoche mistry. The J(HD) value for coordinated H-2 in cis-2 is 12 +/- 3 Hz. T he implication of this small value and of a T1min(200 MHz) of 38 ms is an H/H distance of 1.07-1.35 angstrom, which compares to the neutron diffraction distance of 1.11(3) angstrom. The Ir-H distances of cis-2 are unprecedented in that the hydride-Ir distance (1.584(13) angstrom) is not shorter than the distances to the H-2 hydrogens (1.537(19) and 1.550(17) angstrom). One of the H-2 hydrogens interacts with chloride of an adjacent molecule to give an infinite hydrogen-bonded polymer. An inelastic neutron scattering spectroscopic study on solid IrHCl2(H2 )(PiPr3)2 sets a lower limit on the rotational barrier of the Ir(H2) u nit of 2.0 kcal/mol. Ab initio calculations on IrHCl2(H2)(PH3)2 yield a H-H distance in these two isomers of 0.81 and 1.4 angstrom, respecti vely, showing that the moiety IrHCl2(PH3)2 with chlorides mutually cis is a much stronger reducing agent than that with chlorides trans (and thus H trans to H-2). Crystallographic data: For cis-2 (at 15K), a = 13.008(4) angstrom, b = 11.296 (4) angstrom, c = 16.095(4) angstrom in space group Pna2(1) (Z = 4). For Ir(H)2Cl(PtBu2Ph)2 (at 15K), a = 8.2 36(2) angstrom, b = 17.024(6) angstrom, c = 20.528(10) angstrom, beta = 96.27(4)degrees in space group P2(1)/c (Z = 4).