Coordination Chemistry Reactions of Metal Complexes

Substitution reactions Labile complexes Fast substitution reactions (< few min) Inert complexes Slow substitution reactions (>h) a kinetic concept Not to be confused with stable and unstable (a thermodynamic concept  G f <0)

Mechanisms of ligand exchange reactions in octahedral complexes Dissociative (D)Associative (A) Interchange (I) I a if association is more important I d if dissociation is more important

Kinetics of dissociative reactions

Kinetics of interchange reactions Fast equilibrium K 1 = k 1 /k -1 k 2 << k -1 For [Y] >> [ML 5 X]

Kinetics of associative reactions

Principal mechanisms of ligand exchange in octahedral complexes Dissociative Associative

Dissociative pathway (5-coordinated intermediate) Associative pathway (7-coordinated intermediate) MOST COMMON

Experimental evidence for dissociative mechanisms Rate is independent of the nature of L

Experimental evidence for dissociative mechanisms Rate is dependent on the nature of L

Inert and labile complexes Some common thermodynamic and kinetic profiles Exothermic (favored, large K) Large E a, slow reaction Exothermic (favored, large K) Large E a, slow reaction Stable intermediate Endothermic (disfavored, small K) Small E a, fast reaction

Labile or inert? LFAE = LFSE(sq pyr) - LFSE(oct)

Why are some configurations inert and some are labile? Inert !

Substitution reactions in square-planar complexes the trans effect (the ability of T to labilize X)

Synthetic applications of the trans effect

Mechanisms of ligand exchange reactions in square planar complexes

Electron transfer (redox) reactions M 1 (x+) L n + M 2 (y+) L’ n M 1 (x +1)+ L n + M 2 (y-1)+ L’ n -1e (oxidation) +1e (reduction) Very fast reactions (much faster than ligand exchange) May involve ligand exchange or not Very important in biological processes (metalloenzymes)

Outer sphere mechanism [Fe(CN) 6 ] 4- + [IrCl 6 ] 2- [Fe(CN) 6 ] 3- + [IrCl 6 ] 3- [Co(NH 3 ) 5 Cl] 2+ + [Ru(NH 3 ) 6 ] 2+ [Co(NH 3 ) 5 Cl] + + [Ru(NH 3 ) 6 ] 3+ Reactions ca. 100 times faster than ligand exchange (coordination spheres remain the same) r = k [A][B] Tunneling mechanism

Inner sphere mechanism [Co(NH 3 ) 5 Cl)] 2+ + [Çr(H 2 O) 6 ] 2+ [Co(NH 3 ) 5 Cl)] 2+ :::[Çr(H 2 O) 6 ] 2+ [Co III (NH 3 ) 5 (  -Cl ) Çr II (H 2 O) 6 ] 4+ [Co II (NH 3 ) 5 (  -Cl ) Çr III (H 2 O) 6 ] 4+ [Co II (NH 3 ) 5 (H 2 O)] 2+ + [Çr III (H 2 O) 5 Cl] 2+ [Co II (NH 3 ) 5 (H 2 O)] 2+ [Ço(H 2 O) 6 ] NH 4 +

Inner sphere mechanism Reactions much faster than outer sphere electron transfer (bridging ligand often exchanged) r = k’ [Ox-X][Red] k’ = (k 1 k 3 /k 2 + k 3 ) Tunneling through bridge mechanism