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740 Coordination Compounds

metal cations, which are positively charged and therefore the negative can be handled in daylight. The ligand used
interact favorably with electron donors). A typical metal is thiosulfate in the form of an aqueous (and therefore ion-
complex compound is [Cu(NH 3 ) ](SO 4 ) · H 2 O, which is ized) solution of its sodium salt Na 2 S 2 O 3 ·5H 2 O, so-called
4
made by allowing to crystallize the dark blue solution ob- photographer’s hypo. The silver halide dissolves:
tained from adding an excess of strong ammonia to aque-
ous copper sulfate. AgX + 2(S 2 O 3 ) 2− → [Ag(S 2 O 3 ) 2 ] 3− + X − (3)
The interaction of ligand with metal ion is often
The sodium salt of the anion, bisthiosulfatoargentate(I),
called coordination, and the solid compounds that contain
can be crystallized and was said by its discover. Her-
metal ions complexed by ligands are called coordination
schel, to have a sweet taste. Oddly, the corresponding
compounds. Typical examples are K 2 [PtCl 4 ], K 2 [PtCl 6 ],
compound of the heaviest of the “coinage metals” (copper,
[Pt(NH 3 ) 4 ]Cl 2 , and [Pt(NH 3 ) 6 ]Cl 4 .
silver, gold). Na 3 [Au(S 2 O 3 ) 2 ] · 2H 2 O(1), also has biolog-
ical properties, being used under the name Sanochrysine
in treating rheumatoid arthritis by chrysotherapy.
I. INTRODUCTION
Whenever any metal salt and any Br¨onsted base (an anion
or other molecule with electronegative atoms, such as ni-
trogen or oxygen, that has the capacity to donate electrons)
come into contact, coordination is likely to occur to give a
complex compound. For example, when solid nickel chlo-
ride (yellow) reacts with a stream of ammonia gas, it is
converted to purple hexamminenickel(II) chloride:

NiCl 2 (solid) + 6NH 3 (g) → [Ni(NH 3 ) 6 ]Cl 2 (solid) The gold ion is linearly bonded by two sulfur atoms.
Leather is a complicated material but contains much
Similarly, on the addition of strong ammonia, a base (in

water, NH 3 + OH 2 NH + OH ; all four species are protein; it is always a good ligand, because it contains
+
−
4 peptide linkages, from which either oxygen or nitrogen
present), little by little to a blue aqueous solution of cop-
can donate electrons to a metal ion, forming a coordinate
per(II) sulfate, the final soluble species is the complex
bond, and in which the many functional groups of the side
tetraammine–copper(II) ion; its stability is clear from the
chains of the amino acids may also interact with metal
fact that it is formed by the dissolution [Eq. (2)] of the
ions (as shown by the atoms underlined in structure 2).
intermediate solid [Eq. (1)]. This solid is basic copper(II)
sulfate, known also as several minerals (brocchantite, lan-
gite, wroewulfite) in oxidized sulfide ore zones. The word
basic in the name simply reflects the presence of the hy-
−
droxide ion. OH , the basic constituent of water.
4Cu 2+ (aq) + SO 2− + 6OH →{Cu 4 (SO 4 )(OH) 6 }
−
4
(1)
{Cu 4 (SO 4 )(OH 6 )}+ 16NH 3 → 4[Cu(NH 3 ) 4 ] 2+
This extra element of cross-linking of the polymer
2−
−
+ SO + 6OH (2)
4
(polypeptide) chains by metal ions—in practice, chiefly
The dark blue solution of the tetraammine–copper(II) chromium(III) giving “chrome-leather”—underlies the
species in strong aqueous ammonia (Schweitzer’s solu- utility of leather tanning. It is the formatin of coordinated
tion) will dissolve cellulose. On acidification, the ammo- chromium(III) that gives the tan.
nia is neutralized (protonated forming ammonium ion, Metal ions coordinated by ligands are common, giving
NH ) and the cellulose is reforemed (at one time, through rise to effects of striking beauty and great importance. The
−
4
spinnerets, to make a commercial fiber). changes in the color of blood on oxygenation arise because
When silver halide emulsions are used as photographic of a change in one ligand on the iron ion in the coordination
films, development of the parts exposed to light to give compound hemoglobin. Many examples of chemotherapy
black silver is followed by “fixing,” which is simply metal depend on the formation of coordinated metal ions, as in
complexing. The unchanged (nonimaged) silver halide the removal of the excess of copper from patients with
(usually chloride and bromide) must be removed before Wilson’s disease (hepatolenticular degeneration) using