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Substitution reactions at octahedral complexes: the search for mechanism

Substitution reactions at octahedral complexes: the search for mechanism. Begin by determining whether the intimate mechanism is a or d . Table 1. Aquation refers to the reaction. [Co(NH 3 ) 5 X] n+ + H 2 O  [Co(NH 3 ) 5 (H 2 O)] 3+ + X. Rate constants vary by 6 orders of manitude.

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Substitution reactions at octahedral complexes: the search for mechanism

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  1. Substitution reactions at octahedral complexes: the search for mechanism

  2. Begin by determining whether the intimate mechanism is a or d. Table 1 Aquation refers to the reaction [Co(NH3)5X]n+ + H2O  [Co(NH3)5(H2O)]3+ + X Rate constants vary by 6 orders of manitude  Strongly dependent on the nature of the leaving group Anation refers to the reaction [Co(NH3)5(H2O)]3+ + Y  [Co(NH3)5(H2O)]n+ + H2O Rate constants vary by a factor of 10  Weakly dependent on the nature of the entering group  d activation

  3. Rate more sensitive to the nature of the entering group than the leaving group • anation reactions vary by 3 orders of magnitude • aquation reactions vary by at most 2 orders of magnitude Table 2 - Data for Ru3+  probably under a activation

  4. Table 3 - Data for Co3+ complexes of the type Steric effects If one crowds the metal ion: • speed up reactions under d activation • retard reactions under a activation As bulk of the equatorial ligand increases, so does the rate of the reaction  d activation

  5. Electronic effects If the inert ligands stabilise a 5 coordinate intermediate, and the reaction proceeds faster, then we conclude the reaction is under d activation Table 4 • The saturated complex (cyclam) reacts slowly • bis(dmg) complex reacts faster • -unsaturated, with electron-withdrawing substituents • trans[14]diene reacts fastest • - unsaturated; electron donating group (CH2) on N So, increasing the donation of electron density to the metal ion stabilises the loss of the chloride axial ligand  d activation

  6. H2O NH3 Cl- OH-  donors only low down in the spectrochemical series   donors cis complexes where these are present are quite reactive Table 5 The reactivity of cis versus trans complexes displacement of Cl- by H2O

  7. Cl- departing p orbital of a  donor like Cl- of OH- in the cis position donates electron density into emerging vacant metal orbital This accords with a mechanism under d activation

  8. Cl- departing  donor in the trans position orthogonal orbitals (no net overlap)

  9. rearrange (slow)  donation

  10. We saw in Chapter 3 that... D saturating rate constant = k1 I= k A = only saturates at the diffusion limit Consider an aqua complex. rate of dissociation of departing X interchange rate constant of X and Y

  11. Hence, for a D mechanism, ksat = k1 and the limit is set by the rate of water exchange For an Idmechanism, ksat = k, the rate constant for the exchange of departing H2O and entering Y But [H2O] = 55 M in aqueous solution since [H2O]outer sphere >> [Y]outer sphere, the rate is also limited by the rate of water exchange For an Iamechanism, ksat = k, the rate constant for the exchange of departing H2O and entering Y. But this is dominated by bond forming between entering Y and the metal rate could be greater than the rate of H2O exchange

  12. Table 6 Rh3+ and Ir3+ complexes under associative activation Hence: for d actication, rate cannot be > rate of H2O exchange for a activation, the rate may be greater than the rate of H2O exchange

  13. For d activation: [Cr(H2O)5X]n+ [Cr(H2O)5]m+ + X (X = H2O, OH-) (This is a D process; Idwould have Y involved as X departs.) Effects of charge See Table 7 As the charge on the metal complex increases, the stronger the MX bond  rate decreases Rate is faster when X = H2O (n+ = 3+) than when X = OH- (n+ = 2+) Cr3+ data is in line with a d intimate mechanism

  14. Electrostriction Ordering or disordering of solvent molecules around the metal centre during a chemical reaction Effect is predominantly seen in values of S‡ Charge density has been increased in the transition state S‡ < 0, as the solvent becomes more ordered around the system

  15. The ordering of the solvent is largely unaffected and the contribution to S‡ will be close to zero. There is charge neutralisation in the transition state; the solvent will be less ordered and the electrostriction contribution to S‡ > 0

  16. Corrections for electrostriction effects should be made before any definitive statements concerning mechanism based on values of S‡. After correction for electrostriction effects: S‡ > 0  d S‡ < 0  a S‡  0  no conclusions can be reached

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