Intramolecular exchange of coordinated and dangling phosphines in pentacarbonyl group 6 complexes of 1,1,2-tris(diphenylphosphino)ethane

The linkage isomers, (OC)5M[κ1-PPh2 CH2CH(PPh2)2] 1 and (OC)5M[κ1-PPh2 CH(PPh2)CH2PPh2] 2 (M = Cr, Mo and W) exist in equilibrium at room temperature. Equilibrium constants for 1Cr ⇌ 2Cr, 1Mo ⇌ 2Mo and 1W ⇌ 2W at 25 °C in CDCl3 are 2.61, 5.0 and 4.74, respectively. Enthalpy favors the forward reaction (ΔH = −13.5, −12 and −12.2 kJ mol−1, respectively) while entropy favors the reverse reaction (ΔS = −37.6, −28 and −28.2 J K−1 mol−1, respectively). Isomerization is much faster than chelation with 1Mo ⇌ 2Mo ≫ 1W ⇌ 2W > 1Cr ⇌ 2Cr. Enthalpies of activation for 1Cr ⇌ 2Cr and 1W ⇌ 2W are 119.0 and 92.6 kJ mol−1, respectively, and entropies of activation are 1.4 and −28.2 J K−1 mol−1, respectively. Isomerization is 104 times faster for these complexes than for (OC)5M[κ1-PPh2CH2CH2P(p-tolyl)2]. A novel mechanism is proposed to account for the rate differences. The X-ray crystal structure of 2W shows that the phosphorus atom of the short phosphine arm lies very close to a carbon atom of the W(CO)4 equatorial plane (3.40 Å) which could allow “through-space” coupling, accounting in part for the observation of long-range JPC and JPW coupling. The X-ray structure of (OC)5W[κ1-PPh2 C(CH2)PPh2] 5W has been determined for comparison to 2W.