Addition of Aldehydes and Acyl Chlorides to [Rh(PⁱPr₃)₂Cl]₂. Thermodynamics and Molecular and Crystal Structures of Rh(Pⁱ Pr₃)₂ CIX[C(O)PH] (X = H, Cl)

Addition of aldehyde (RCHO; R = n-octyl, Ph, p-Tol, p-MeOC₆H₄, P-CF3C6H4) to [Rh(PⁱPr₃)₂Cl]₂ (1) results in rapid addition of the aldehyde C—H bond to yield Rh(PⁱPr₃)₂-CIH[C(O)R] (2-R). 2-Ph was isolated, and a single-crystal X-ray diffraction study reveals a trigonal-bipyramidal structure with a small H—Rh—C(acyl) angle of 85(4)°. Enthalpies of addition to 1 were measured by solution calorimetry (R, ΔH/(kcal/mol)): octyl, -15.2 ± 0.3; Ph, -10.8 ± 0.4; p-Tol, -10.6 ± 0.4; p-CF₃C₆H₄, -12.7 ± 0.4; p-MeOC₆H₄, -10.5 ± 0.3. Electron-withdrawing para substituents on the aromatic aldehydes favor addition. Addition of nonanal is more favorable than addition of benzaldehydes, probably due to steric effects, particularly the close hydride-phenyl contact found in 2-Ph. 1 reacts with acyl chlorides (RC(O)Cl, R = octyl, Ph) rapidly to give Rh(PⁱPr₃)₂Cl₂[C(O)R] (3-R). 3-Ph possesses a squarepyramidal structure. The enthalpies of addition were also measured calorimetrically (R, ΔH/(kcal/mol)): octyl, -24.6 ± 0.3; Ph, -21.7 ± 0.3. Relative to the addition of acyl chlorides, the exothermicity of aldehyde addition is greater than would be expected on the basis of thermodynamic data for related late-transition-metal complexes.