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Reactivity of Complex

compounds

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Basic Principles ofReaction

In general, chemical reactions move from one

energy minimum (the reactants) through a

higher energy structure (the transition

state) to another energy minimum (theproducts).

In simple cases, the energies and bond distancescan be shown as a three-dimensional surface,

with two different bond distances along the base-plane axes and free energy as the verticaldimension

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The reaction MX + Y MY + X begins ata point representing the short M-X distanceof the bond to be broken and the longerdistance between the two reactants MX andY.

As the M-X bond breaks and the M - Ybond forms, the reaction point moves torepresent the short M-Y bond distance andthe longer distance between the two productsMY and X.

The principles ….. 

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The free energy surface usually has a saddle shape,much like a mountain pass between two valleys.

For more complex reactions, such a visualrepresentation is difficult or impossible, but the path

between the reactants and the products is always thelowest energy pathway and must be the sameregardless of the direction of the reaction.

This is the principle of microscopicreversibility, frequently described by themountain pass analogy; the lowest pass going inone direction must also be the lowest pass going inthe opposite direction.

The principles ……. 

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If the reaction is such that the conversionfrom reactants to products takes place withno hesitation at the transition point as in

Figure 1 (a), the structure at that state iscalled the transition state.

If there is a structure that lasts a bit longer

as in Figure 1 (b), and particularly if it isdetectable by some experimental means, it is

called an intermediate.

The principles ……. 

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Figure Energy 1. Profiles and Intermediate Formation.

(a) No intermediate. The activation energy is the energydifference between the reactants and the transition state.

(b) An intermediate is present at the small minimum at thetop of the curve. The activation energy is measured at the

maximum point of the curve

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Thermodynamic stability is expressed bythe words stabile and unstabile

Kestabilan kinetik dinyatakan dengan

istilah INERT (LEMBAM) dan LABIL Jadi suatu senyawa kompleks yang

bersifat stabil (termodinaik) mungkin

bersifat labil atau mungkin bersifatinert, demikian juga senyawa komplekstak-stabil mungkin juga bersifat inert 

walaupun biasanya bersifat labil.

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Kinetic inertness/lability

Many synthetic reactions require substitution, orreplacing one ligand by another.

This is particularly true when the starting material isin aqueous solution, where the metaI ion is likely tobe in the form [M(H2O)m]n+.

Some simpler reactions of this type produce coloredproducts that can be used to identify metal ions:

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These reactions, and others like them, are very fastand form species that can undergo a variety ofreactions that are also very fast.

Addition of HNO3(H+), NaCI(Cl-), H3PO4(PO4

3-),

KSCN(SCN-), and NaF(F-) successively to a solutionof Fe(NO3)3.9H2O shows this very clearly.

The initial solution is yellow because of the presenceof [Fe(H2O)5(OH)]2+ and other "hydrolyzed" species

containing both water and hydroxide ion. Although the exact species formed in this series

depend on solution concentrations, the products inthe reactions given here are representative:

Kinetic inertness/lability

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Lability Compounds such as these that react rapidly

are called labile.

In many cases, exchange of one ligand for

another can take place in the time of mixingthe solutions.

Taube has suggested a reaction half-life (thetime of disappearance of half the initial

compound) of one minute or less as thecriterion for lability.

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Inertness Compounds that react more slowly are

called inert or robust (a term used less

often). An inert compound is not inert in the

usual sense that no reaction can take place; itis simply slower to react.

These kinetic terms must also bedistinguished from the thermodynamic terms

stable and unstable.

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Senyawa Inert (lembam) A species such as [Fe(H2O)5(F)]2+ is very

stable (has a large equilibrium constant forformation), but it is also labile.

On the other hand, hexaaminecobalt(3+) isthermodynamically unstable in acid and candecompose to the equilibrium mixture on theright but it reacts very slowly (has a very high

activation energy) and is therefore called inertor robust.

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Senyawa Inert (lembam) The possible confusion of terms is

unfortunate, but no other terminologyhas gained general acceptance.

One possibility is to call the compoundssubstitutionally or kinetically labile orinert, but these terms are not in general

use at this time.

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Although there are exceptions, general rules canbe given for inert and labile electronic structures.

Inert octahedral complexes are generally thosewith high ligand field stabilization energies

specifically those with d3

or low-spin d4

throughd6 electronic structures.

Complexes with d8 configurations generally reactsomewhat faster, but slower than the d7, d9, or

d

l0

compounds. With strong-field ligands, d

8

 atoms form square-planar complexes, many ofwhich are inert.

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Compounds with any other electronic structures

tend to be labile. Summarizing, we get:

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Mechanism of ligand substitution

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