Coordination Chemistry Reactions of Metal Complexes NIKAM N

Coordination Chemistry Reactions of Metal Complexes NIKAM N. D. DEPARTMENT OF CHEMISTRY

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 DGf <0)

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

Kinetics of dissociative reactions

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

Kinetics of associative reactions

Principal mechanisms of ligand exchange in octahedral complexes Dissociative Associative

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

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 Ea, slow reaction Stable intermediate Endothermic (disfavored, small K) Small Ea, 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 -1 e (oxidation) M 1(x+)Ln + M 2(y+)L’n M 1(x +1)+Ln + M 2(y-1)+L’n +1 e (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 - + [Ir. Cl 6]2[Co(NH 3)5 Cl]2+ + [Ru(NH 3)6]2+ Reactions ca. 100 times faster than ligand exchange (coordination spheres remain the same) r = k [A][B] Tunneling mechanism [Fe(CN)6]3 - + [Ir. Cl 6]3[Co(NH 3)5 Cl]+ + [Ru(NH 3)6]3+

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(m-Cl)Çr. II(H 2 O)6]4+ [Co. II(NH 3)5(m-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+ [Ço(H 2 O)6]2+ + 5 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