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Encyclopedia of Physical Science and Technology EN003D-147 June 13, 2001 22:58

750 Coordination Compounds

TABLE VI Mixed Coordination Spheres on Cobalt(III) and Isolating the less thermodynamically stable of two in-
Chromium(III) terconvertible forms of the same composition (whether it
a
ν log ε be coordination compounds, allotropes of elements, or any
other chemical composition) can be done only if the rate of
Compound Cobalt Chromium Color
reaching equilibrium is so slow as to render the conversion
[M(NH 3 ) 6 ] 3+ 20.7 1.8 21.0 1.6 Orange of the metastable isomer to the stable one very protracted.
[M(NH 3 ) 5 Cl] 2+ 19.4 1.75 19.5 1.6 Pink This is a form of Ostwald’s law of metastable intermedi-
trans-[M(NH 3 ) 4 Cl 2 ] + 16.1 1.4 16.5 1.4 Green ates. Such rates are slow (minutes < t 1/2 < years) for the
cis-[M(NH 3 ) 4 Cl 2 ] + 19.1 1.9 19.2 1.9 Green equilibrations of coordinated cobalt(III), chromium(III),
6
a few (spin-paired) d ferrous compounds, such as salts
a −1
In kilokaysers (1 kK = 1000 cm ). of the ferroin 22a, tris-1,10-phenanthrolineiron(II) cation,
and a few octahedral nickel(II) species with strong field
8
G. Kinetic Properties of Coordination ligands (3d , e.g., the tris-1,10-phenanthrolinenickel(II)
Compounds ion 22b).
Reactions of coordination compounds can be divided into
several classes, depending on whether the oxidation state
of any atom changes during the transformation of start-
ing materials (factors) into products. No change gives a
reaction such as Eq. (49).
[Co(NH 3 ) 5 Cl] 2+ + OH 2 → [Co(NH 3 ) 5 (OH 2 )] 3+ + Cl −
(49)
Many formations and decompositions or other equili-
brations of coordination compounds are extremely rapid.
The half-life of a reaction such as the replacement (“sub-
stitution”) by ammonia of water coordinated to nickel(II)
ions is typically microseconds to milliseconds, and there
is indeed a convenient distinction (due to Taube) for re-
actions in solution between kinetically labile and kineti-
cally inert systems. On mixing 0.1 M aqueous solutions of
the reagents, labile equilibria are fully established within
1 min, whereas inert systems take longer. Many of the
ions of the heavier (second and third row) transition el- Oxidations and reductions are common and important,
ements in several oxidation states (e.g., both Pt 2+ and chiefly because coordination compounds of the transition
6
Pt ) are inert, as are many spin-paired d ions (Fe , metals may readily pass (often rapidly) from one oxida-
2+
4+
4+
Co ,Ni ) and chromium(III) in the first row. Kinetic tion state to another and because one-electron changes
3+
lability in solution is the rule for coordination compounds are common (whereas elsewhere in the periodic table, this
containing main group metals. Reactions of solid coordi- would involve free-radical formation). For example, sev-
nation compounds (like most other solid-state changes) eral named organic reagents utilize half-cells based on
are usually slow. It is this kinetic inertness that has coordination compounds.
led to the isolation of so many metastable coordination Typically, Decker’s reagent is alkaline ferricyanide
compounds. [hexacyanoferrate(III)], which may be used to oxidize the
The decomposition (via substitution) of hexaam- pseudobase 23 to the pyridone 24, as in Eq. (52).
minecobalt(III) salts in acidic water [Eq. (50)] is thermo-
dynamically very favorable; that is, K in Eq. (51) is very
large, but the rate is extremely small. Solutions of such (52)
hexaamminecobalt(III) salts in dilute acid are unchanging
for weeks.

+
[Co(NH 3 ) 6 ] 3+ + 6H 3 O [Co(OH 2 ) 6 ] 3+ + 6NH +

4
Many other such reactions occur with changes by a com-
(50)
bination of oxidation–reduction and substitution. When

+ 6 6
+
K = [Co(OH 2 ) 6 ] NH [Co(NH 3 ) 6 ] H 3 O + (51) Tollens’s reagent, [Ag(NH 3 ) ] , oxidizes aldehydes, the
4 2