EQUILIBRIUM STUDIES OF PHOSPHINE AND AMINE SUBSTITUTION REACTIONS OF TUNGSTEN CARBONYL

Equilibrium constants, K, have been determined for the reaction (Am)W(CO)5 + L ⇄ (L)W(CO)5 + Am. For a given L group, K values increase for different amines, Am, in the order: p-CH3OC6H4NH2 < p-CH3C6H4NH2 < C6H5NH2 < p-Br-C6H4NH2. This trend is interpreted to mean that the strength of the W-Am bond decreases with decreasing basicity of the amine. For a given Am group, K decreases with different L groups as follows: P(n-C4H9)3 ~ P(C6H11)3 ≥ P(OCH2)3CC2H5 > P(SCH2)3CCH3 > P(O-n-C4H9)3 > P(C6H5)3 > As(C6H5)3 > Sb(C6H5)3 ~ P(OC6H5)3 ~ p-CH3OC6H4NC > Bi(C6H5)3. Since this is essentially the order of decreasing basicity of the phosphorus donor ligands, the results suggest that bonding, not tt bonding, is largely responsible for the strength of the W-L bond. Enthalpies and entropies have been determined for some of these reactions, and they support the above conclusions. © 1969, American Chemical Society. All rights reserved.