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Reaction Mechanisms of D-Orbital Complexes

The document summarizes reaction mechanisms for d-orbital complexes, including ligand substitution and oxidation-reduction reactions. Ligand substitution in octahedral complexes can occur via dissociative (D) or associative (A) mechanisms. Oxidation-reduction reactions can be either outer-sphere, where electron transfer occurs between complexes without disrupting the coordination sphere, or inner-sphere, where electron transfer involves bridging ligands between metal centers. Inner-sphere electron transfer in octahedral complexes is proposed to proceed via formation of an intermediate with a bridging ligand between the reduced and oxidized metal ions.

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38 views21 pages

Reaction Mechanisms of D-Orbital Complexes

The document summarizes reaction mechanisms for d-orbital complexes, including ligand substitution and oxidation-reduction reactions. Ligand substitution in octahedral complexes can occur via dissociative (D) or associative (A) mechanisms. Oxidation-reduction reactions can be either outer-sphere, where electron transfer occurs between complexes without disrupting the coordination sphere, or inner-sphere, where electron transfer involves bridging ligands between metal centers. Inner-sphere electron transfer in octahedral complexes is proposed to proceed via formation of an intermediate with a bridging ligand between the reduced and oxidized metal ions.

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rejie magnaye
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Reaction Mechanisms of

d-orbital Complexes
Ligand Substitution Reactions:
Octahedral Complexes
Inert & Labile Complexes
– labile => exchange t1/2 less than 1 min. at RT
– inert => exchange t1/2 more than 1 min. at RT
Oh Transition Metal Complexes
• if d e-s are in eg*
– replacement easy for d4, d5, d6
– weak M - L bond
• if less than 3 e-s in t2g
– replacement easy for d0, d1, d2
– no large repulsion energies
Octahedral Complexes
Substitution Reactions
• D mechanism => complete ligand
dissociation
• A mechanism => complete ligand
association
Reaction Profiles

dissociative interchange associative


D Mechanism, SN1
slow
ML6 --------> ML5 + L
k1
- d[ML6]
rate = ------------ = k1 [ML6]
dt
fast
ML5 + L' -------> ML5L'
ML5: trigonal bipyramidal or sqaure pyramidal
D Mechanism, SN1
limiting case of more general
k-1
ML6 ====== ML5 + L
k1
k2
ML5 + L' -------> ML5L'
- d[ML6] k1k2[ML6][L']
rate = ----------- = ------------------
dt k-1 + k2[L']
D Mechanism, SN1
limiting case of more general
k-1
ML6 ====== ML5 + L
k1
k2
ML5 + L' -----> ML5L'

- d[ML6] k1k2[ML6][L']
rate = ----------- = --------------------
dt k-1 + k2[L']
if [L'] is very large, then k2[L'] >> k-1
D Mechanism, SN1
limiting case of more general
- d[ML6] k1k2[ML6][L']
rate = ------------- = ------------------
dt k-1 + k2[L']
if [L'] is very large, then k2[L'] >> k-1, thus

rate = k1 [ML6]
A Mechanism, SN2
slow
ML6 + L' -----> ML6L'
k
- d[ML6]
rate = ----------- = k[ML6][L']
dt
fast
ML6L' ------> ML5L' + L

ML7: pentagonal bipyramidal, L' axial or


equatorial
Ligand Substitution Reactions:
Octahedral Complexes

Oxidation-Reduction Reactions

• inner sphere
– M - L - M'
• outer sphere
– coordination sphere intact, e-s jump from one
metal to another
Outer Sphere, Oh
1.rate depends upon 2 concentrations
2.rate e- transfer significantly faster than
ligand substitution
-e- oxidation
| |
[Co(NH3)5Cl]+2 + [Ru(NH3)6]+2 -----> [Co(NH3)5Cl]+ + [Ru(NH3)6]+3
| +e- reduction |
Inner Sphere, Oh
[Co(NH3)5Cl]+2 + [Cr(H2O)6]+2
H+
----->
H2O
[Co(H2O)6]+2 + [Cr(H2O)5Cl]+2 + 5NH4+
Inner Sphere, Oh
proposed reaction intermediate

[(NH3)5Co---Cl---Cr(H2O)5]+4

36Cl-free ion in solution, little seen in final


[Cr(H2O)5Cl]+2
Inner Sphere, Oh
[Cr(H2O)6]+2 -----> [Cr(H2O)5]+2 + H2O
labile labile
Inner Sphere, Oh
immediately before electron transfer

[Co(NH3)5Cl]+2 + [Cr(H2O)6]+2
----->
[(NH3)5CoIII--Cl---CrII(H2O)5]+4
Inner Sphere, Oh
electron transfer

[(NH3)5CoIII--Cl---CrII(H2O)5]+4
<=====>
[(NH3)5CoII--Cl---CrIII(H2O)5]+4
Inner Sphere, Oh
immediately after electron transfer

[(NH3)5CoII--Cl---CrIII(H2O)5]+4
----->
[Co(NH3)5]+2 + [Cr(H2O)5Cl]+2
labile inert
Inner Sphere, Oh
[Co(NH3)5]+2 + 5H+ + 6H2O
----->
[Co(H2O)6]+2 + 5NH4+
Trans-Effect in Square Planar Complexes
Trans-Effect in Octahedral Complexes

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