CHEM 511 chapter 21 page 1 of 7 Chapter 21 Coordination chemistry: reactions of complexes Reactions of Complexes Typically measure ligand substitution reactions in solution (usually water) Lability and Inertness Labile: complexes with half-lives under 1 minute Inert: complexes with half-lives longer than 1 minute (better term is non-labile) Figure 20.1 shows lifetimes for exchange of water Generalizations 1. All complexes of s-block elements are extremely labile, except Be 2+ and Mg 2+ (Why not Be 2+ /Mg 2+ )?) 2. All M 3+ ions of the f-block elements are very labile 3. d 10 ions with low oxidation numbers are very labile (Why?) 4. M 2+ complexes are moderately labile, and Cu 2+ is very labile (Why for Cu 2+?) 5. M 3+ complexes are less labile than M 2+ (Why?) 6. d 3 and low-spin d 6 octahedral complexes of first row d-metals are inert (nonlabile), chelates are particularly nonlabile (Why? Why?) 7. In 4d and 5d metals, complexes are usually inert (Why?)
CHEM 511 chapter 21 page 2 of 7 Lifetime of complex is also dependent on the incoming ligand, thus we define the term nucleophilicity: the rate of attack on a complex by a given Lewis base relative to the rate of attack by a reference Lewis base. Mechanisms of Substitution Three Main Types Dissociative SN1 mechanism Depends on leaving group Intermediate possibly detected? Associative SN2 mechanism Depends on leaving group AND nucleophile Intermediate possibly detected? Interchange No true intermediate exists the leaving group and entering group exchange in a single step by forming an activated complex, but not a true intermediate Evidence for associative vs. intermediate mechanism: 1. Is the intermediate observed in other, similar reactions?
CHEM 511 chapter 21 page 3 of 7 2. Is there a change in stereochemistry? Consider cis-[ptl2(pr3)2] transformed to trans-[ptlx(pr3)2] This process happens in many trigonal bipyramidal complexes. How can this process be stopped or slowed down?
CHEM 511 chapter 21 page 4 of 7 Rate Determining Step (rds) Rate laws are dependent on the slowest step in the mechanism and each type of mechanism involves formation of one bond and breaking of another. A mechanism may be associative, but the bond breaking step may be the rds. Aa, Da mechanisms Ad, Dd mechanisms Ia, Id mechanisms Look for dependence on incoming ligand (Y). Associative mechanisms are very dependent on Y Dissociative mechanisms are largely independent of Y Square planar substitutions Generally expect associative mechanism for these structures (why?) but sometimes it is more complicated. The trans effect: the effect of a spectator ligand upon the rate of substitution of ligands opposite to it (i.e., the trans position). This is a kinetic factor only doesn t change which ligand will substitute
CHEM 511 chapter 21 page 5 of 7 Two influences: ground state (σ-bonding) and transition state (π-bonding) Ligands trans to each other use the same metal orbitals to bond a stronger σ-donor on one side, weakens the ligand opposite it. σ-donor: OH - < NH3 < Cl - < Br - < CN -, CO, CH3 - < I - < SCN - < PR3 < H - In the transition state, strong π-acceptors will pull incoming electron density out of the metal to stabilize the metal π-acceptor: Br - < I - < NCS - < NO2 - < CN - < CO, C2H4 Polarizability of ligands can also play a role: Cl - < Br - < I - EX. Starting with [Pt(NH3)4] 2+ what happens in HCl? Ex. Starting with [PtCl4] 2- what happens in NH3?
CHEM 511 chapter 21 page 6 of 7 Steric effects on square planar complexes Bulky ligands, especially cis, will slow down reaction rates Data for cis-[ptcll(pet3)2] +, where Cl - is exchanged for H2O L Pyridine 2-methylpyridine 2,6-dimethylpyridine Rate constant (s -1 ) 8 10-2 2.0 10-4 1.0 10-6 What type of mechanism does this support (A or D)? Stereochemistry of square planar Substitution usually keeps the same geometry, though if the transition state lifetime is long, pseudorotation may occur Only long-lived associative mechanism intermediates will form the necessary 5-coordinate species to be stereomobile.
CHEM 511 chapter 21 page 7 of 7 Redox chemistry between complexes Two main mechanisms for inorganic, aqueous species. Outer sphere electron transfer Minimal change occurs with the coordination of the redox centers Occurs when complexes are inert towards ligand substitution. EX. Fe II (CN) 6 4- + Ir IV Cl 6 2- Fe III (CN) 6 3- + Ir III Cl 6 3- Ligands can affect rate of transfer (Fe 2+ + Co 3+ Fe 3+ + Co 2+ ) Fe(H 2O) 6 2+ + Co(H 2O) 6 3+ k(rate) = 10 Fe(o-phen) 3 2+ + Co(H 2O) 6 3+ k(rate) = 1.4 10 4 o-phen is Inner sphere electron transfer The two complexes form a bridged species Involves a change in the inner coordination sphere Co III Cl(NH 3) 2+ 5 + Cr II (H 2O) 2+ 6 + 5H 2O + 5H + Co II (H 2O) 2+ 6 + Cr III Cl(H 2O) 2+ + 5 + 5NH 4 Ligand effect HO 2C-CH=CH-CO - 2 k(relative rate) = 10 7 HO 2CCH 2CH 2CO - 2 k(relative rate) = 1 Also, I - > Br - > Cl -