A theoretical study of [M(PH3)4] (M = Ru or Fe), models for the highly reactive d8 intermediates [M(dmpe)2] (dmpe = Me2PCH2CH2PMe2). Zero activation energies for addition of CO and oxidative addition of H2‡
Density functional calculations have been carried out on [M(PH3)4] species as models for transient [M(dmpe)2] formed from the photolysis of [M(dmpe)2H2] (M = Ru or Fe, dmpe = Me2PCH2CH2PMe2). Calculations have also been performed on [Rh(PH3)4]+ as a model for the relatively inert [Rh(dmpe)2]+. The singlet electron configurations of [Ru(PH3)4] and [Rh(PH3)4]+ were found to have D2d geometries with trans P–M–P angles of 159 (M = Ru) and 172° (M = Rh+). Singlet [Fe(PH3)4] was computed to have a C2v structure with trans P–M–P angles of 137 and 160° at Fe. The triplet configurations of [Fe(PH3)4] and [Ru(PH3)4] were predicted to adopt C2v geometries with angles of ca. 155 and 95° for both species. Singlet [Ru(PH3)4] is calculated to be 11.7 kcal mol–1 more stable than the triplet, but the triplet form of [Fe(PH3)4] is the more stable by 8.0 kcal mol–1. The addition of CO and oxidative addition of H2 to [M(PH3)4] (M = Ru or Fe) were calculated to be highly exothermic. In contrast, the reaction between [Rh(PH3)4]+ and H2 is less thermodynamically favoured, consistent with the lower reactivity of experimental Rh+ analogues. Both the oxidative addition of H2 and addition of CO were calculated to proceed without activation energy for [Ru(PH3)4], but only once the ‘end-on’ approach of H2 and an angled approach of CO at long ruthenium–substrate separations are considered. The calculations on [Ru(PH3)4] also reproduced the UV/VIS spectrum and geometry of [Ru(dmpe)2] satisfactorily. The reaction of singlet [Fe(PH3)4] with CO was calculated to be barrierless, while the oxidative addition of H2 required a very small activation energy (≈1 kcal mol–1) at long Fe–H2 distances. The reaction of [Rh(PH3)4]+ with H2 has a somewhat larger activation barrier (≈3 kcal mol–1) and is predicted to pass through a product-like C2v transition state.