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

Coordination Compounds 751

product is silver metal. In Fehling’s solution (which he pK a = 2.7, comparable to that of monochloracetic acid!).
originally called Barreswils’s solution) the oxidant (for re- Eventheaciddissociationofammoniatogiveitsconjugate
ducing sugars) is a complex compound of copper(II) with base is said to become perceptible when it is attached to a
tartrate ions, and in Benedict’s solution it is a complex highly charged metal ion, as in Eq. (56).
with citrate ions. Sarett’s reagent for oxidizing alcohols to
[Pt(NH 3 ) 6 ] 4+ (aq) [Pt(NH 3 ) 5 (NH 2 )] 3+ + H (aq)
+

aldehydes is a coordination compound of chromium(VI),
Cr(C 5 H 5 N) O 3 , where the chromium(VI) becomes re- (56)
2
duced. There are many minor variations of this reaction. Certainly such proton transfers have extremely large rate
Often, the compound to be oxidized is made a ligand, constants.
and the oxidation can then be intramolecular, as in the Dow In water, nearly all substitutions into a coordination
phenol process [Eq. (53)], where the benzoate of basic sphere (i.e., of one ligand for another) have a very simple
copper(II) benzoate is oxidized to salicylate by hydroxide rate equation:
and then carbon dioxide is eliminated to yield phenol.
Rate = k[complex] (57)
C 6 H 5 COOCuOH → C 6 H 5 OH + CO 2 + Cu 0 (53)
That is true whether the rate constant k is large (labile)
A degradation of coordinated salicylate [25, Eq. (54)] is or small (inert), but tells us nothing about the detailed
alsotypical.Thischelatingcontraction[25(six-membered mechanism.
ring) → 26 (five-membered ring)] occurs readily in acid There are a few situations in which the rate equation is
a little more interesting than Eq. (57); for example:

8
1. Substitution at platinum(II) [(5d) ]; four-coordi-
nated square species. Here, often,
Rate = k[complex][incoming nucleophile] (58)
2. “Base hydrolysis” (substitution by hydroxide ion) of
(54) coordination compounds of cobalt(III) with ligands con-
taining an N–H group. The unusual rate equation is
permanganate. Here is another side to the coin: The metal
−
ion, in this case cobalt(III), somehow prevents the oxalate Rate = k[complex][OH ] (59)
ion bound to it in 26 from undergoing its normal (“high
This is commonly thought to imply the presence of a re-
school”) oxidation by permanganate. Such “shielding” ef-
active conjugate base, typically via Eqs. (60)–(63):
fects (loss of normal reactions) on coordination are well
known. [Co(NH 3 ) 5 Cl] 2+ + OH −
Such modifications of the chemistry of ligands attached
+
→ [Co(NH 3 ) 4 (NH 2 )Cl] + H 2 O (60)
to metal ions are becoming increasingly important. Not all
are oxidation or reduction. [Co(NH 3 ) 4 (NH 2 )Cl] → [Co(NH 3 ) 4 (NH 2 )] 2+ + Cl −
+
The high-field, spin-paired nitrosopentacyanoferrate
(61)
ion [Fe(CN) 5 (NO)] , often used as an aqueous so-
2−
lution of its salt—so-called sodium nitroprusside, [Co(NH 3 ) 4 (NH 2 )] 2+ → [Co(NH 3 ) 4 (NH 2 )(OH 2 )] 2+
Na 2 [Fe(CN) 5 NO] · 2H 2 O—undergoes many reactions of
(62)
analytical importance, chiefly as qualitative tests, in which
the nitroso ligand becomes modified, usually without de- [Co(NH 3 ) 4 (NH 2 )(OH 2 )] 2+ + H 2 O
taching from the iron. Examples are the Gmelin test for
→ [Co(NH 3 ) 5 (OH 2 )] 3+ + OH − (63)
sulfur in organic matter (Lassaigne sodium fusion to give
sulfide, which gives a strong purple color with the FeNO This gives Eq. (64) [with the loss of chloride from the con-
unit) and the Bodlander reaction with sulfite to give a jugate base, Eq. (61), as the slow-rate-determining step]:
bright red color.
Evens so simple a reaction as acid–base equilibrium in Rate = k[Co(NH 3 ) 4 (NH 2 )Cl] 2+ (64)
the ligands is strongly modified by metal ions. In Eq. (55), −
= kK[Co(NH 3 ) 5 Cl][OH ] (65)
[L 5 M(OH 2 )] n+ [L 5 M(OH)] (n−1)+ + H + (55)

The base hydrolyses of the much studied cobalt(III) com-
the acid dissociation to form proton and the conjugate pounds are dominated by this type of equation, whereas
base may be strong (e.g., for L = H 2 O, M = Fe, n = 3, the coordination compounds of the equally polarizing