Ch 12 Inorganic Reaction Mechanisms

1. Ch 12 Inorganic Reaction Mechanisms • Introduction • Parallels to Organic Chemistry • Many terms and concepts are the same as in organic mechanisms • Complex geometries are more common in inorganic complexes • More rearrangements are possible • More isomers are possible • Not all metal ions react alike; all carbon atoms do • History and Goals • Werner and Jorgenson discovered many of the basic reactions • Experimentation over many years has yielded proposed mechanisms • Mechanisms can’t be proven, only disproven • We can’t directly observe individual molecules react • Evidence either supports a mechanism or rules it out • Goal: synthesize the predicted products by choosing the appropriate reaction conditions. • Types of Reactions: Substitution, Oxidation/Reduction, Ligand Reactions

2. Substitution Reactions • Inert and Labile Complexes • Labile Complexes = those undergoing substitution with t½ < 1 minute • Many analytically useful reactions are labile substitutions [Cu(H2O)6]2+ + NH3[Cu(NH3)4(H2O)2]2+ + H2O [Fe(H2O)6]3+ + SCN- [Fe(H2O)5(SCN)]2+ + H2O b) Lability is generally a function of the metal ion, not the ligands [Fe(H2O)5(OH)]2+ + H+[Fe(H2O)6]3+ [Fe(H2O)6]3+ + Cl- [Fe(H2O)5Cl]2+ [Fe(H2O)5Cl]2+ + PO43-Fe(H2O)5PO4 Fe(H2O)5PO4 + SCN-[Fe(H2O)5(SCN)]2+ [Fe(H2O)5(SCN)]2+ + F- [Fe(H2O)5F]2+ • Often, the reaction happens at the diffusion limit = as soon as the reactants are mixed, they are done reacting. Diffusion = 1011 s-1 • Labile Metal ions = those with small or zero LFSE • d1, d2, d7, d9, d10 • High spin d4-d6

3. Inert Complexes = those undergoing substitution with t½ > 1 minute • Inert does’t mean unreactive; Inert doesn’t mean thermodynamically stable • Inert does mean slow to react (also known as Robust) [Fe((H2O)5F]2+ = labile, but it is very thermodynamically stable [Co(NH3)6]3+ = inert, but thermodynamically unstable • Inert complexes react slowly, so their products can be isolated and studied • Inert metal ions = those with large LFSE • d3, low spin d4-d6 • Low spin d8 e) High spin (weak field) d8 metals are intermediate in lability • The Dissociative (D) Substitution Mechanism • The mechanism is essentially the same as SN1 in organic chemistry • Dissociation of one ligand results in an isolatable (rarely) intermediate • The new ligand binds at the open coordination site

4. 2) The kinetic analysis a) The Steady State Hypothesis • Intermediates are high energy species (5-coord metal ion) • They react almost as soon as they are formed • Their concentrations are small and constant over most of the course of a reaction b) The D rate Law

5. The Interchange (I) Substitution Mechanism • The mechanism involves the incoming ligand (Y) in the rate determining step • Y can weakly assist the leaving ligand (X) = Dissociative Interchange (ID) • Y can strongly begin bond formation before X leaves = Associative Interchange (IA) • The kinetic analysis • Assumptions to simplify the analysis • [Y] = very large, [Y]0 = [Y] • k2 << k-1 which means reaction #1 is in equilibrium with K1 = k1/k-1 • [M]0 = [ML5X] + [ML5X•Y] • Steady State Equation

6. Comparison of D and I Rate Laws • We can rewrite the I and D rate laws for comparison: • If [Y] is small, both become first order in [M] and [Y] • For D, we get • For I, we get

7. 3) If [Y] is large, both become first order expressions For both I and D, we get • These similarities make it difficult to ever tell these two mechanisms apart based on experimental data • Usually, we vary the concentration of [M], [Y], or [X] to find the order • Isolation of ML5 is proof of the D mechanism • The Associative (A) Substitution Mechanism • The mechanism is essentially the SN2 mechanism from Organic chemistry • Y and X are both partially bonded to M at the transition state • An Intermediate is only rarely isolated • Most reaction best fit between D and A, into the ID and IA mechanisms

8. Experimental Evidence for the Octahedral Substitution Mechanism • The usual mechanism for Octahedral complexes is Dissociation • Even if a reaction is thermodynamically downhill, a large Ea will make it slow • Thermodynamically uphill reactions won’t occur even if Ea is small • Ligand Field Activation Energy = LFAE = difference in ligand field stabilization energy between the octahedral complex and the 5-coordinate intermediate weakly supports the assignment of labile and inert metal ions 5) Oxidation State: higher charge = slower reaction due to greater ligand attraction 6) Ionic Radius: smaller ionic radius = slower reaction due to greater ligand attraction 7) Other Evidence for Dissociative Mechanism • Incoming ligand identity (Y) has no effect on rate • Bulky X increases the rate • DVa= volume of activation is positive for octahedral substitutions because one molecule splits into two at the intermediate

9. LFAE = LFSE(Sq Pyr)– LFSE(Oh)

10. The Associative Substitution Mechanism and Octahedral Complexes • This mechanism is sometimes observed, but is rare • If the identity of Y influences the rate, that suggests Association • If DSa is negative (molecules coming together), that suggests Association

11. The Conjugate Base Mechanism (SN1CB) of Substitution (May look like A mech.) 1) This mechanism requires a deprotonatable ligand on the complex (NH3, H2O) • It also requires presence of hydroxide OH- in aqueous solutions • Mechanism: • Deprotonation lowers the charge on the complex ion, so X- leaving is easier • Evidence • H exchange on ammonia ligands is well known • RNH2 is faster than NH3, because steric crowding favors dissociation • R3N ligands completely stop the reaction (no ionizable protons) 6) The position trans to X is usually the one deprotonated. The trigonal bipyramidal intermediate is more easily achieved.