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‡

Abstract

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 D_2d geometries with trans P–M–P angles of 159 (M = Ru) and 172° (M = Rh⁺). Singlet [Fe(PH₃)₄] was computed to have a C_2v 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 C_2v 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 C_2v transition state.