CHAPTER IV.
COMPOUNDS CONTAINING A COMPLEX NEGATIVE
COMPONENT.THE modern conception of complex compounds arose from the necessity of
classifying the compounds formed by the union of two simple salts. Those
composite salts which in aqueous solution are dissociated entirely into the
simple salts, or their ions, are classed as double salts; those which, instead of
dissociating into the simple salts, give characteristic ions of their own -— for
example, simple metal cations and composite metal-containing anions — are
classed as complex salts:KMgCl 3 = KCI + MgCI 2 = K+ + Mg++ + 3 Cl~ (double salt),
K 2 [HgI 4 ] = 2 K++ [HgIJ~~ (complex salt).In such a complex compound, the composite part which remains intact
during dissociation is known as a complex radical or complex ion ' and may be
regarded as the negative component of a new acid; sometimes indeed the com-
plex acid can be prepared in a free state, for example, hydroferrocyanic acid
(No. 108). The individual constituents of a complex ion, since they are not
present in the free state, do not show all the reactions which are characteristic
of the simple ions; it is this reduced ability of metals contained in complex
radicals to react, which forms the most important criterion of this class of
compounds.
The conception of complex compounds has, however, extended considerably
beyond the range of these composite salts. In the first place, the distinction
between double and complex salts is merely a qualitative one. Transition
forms are known in which the simple salts and their ions as well as the complex
ions are found among the dissociation products; the nature and extent of the
dissociation is also dependent to a marked degree upon the nature of the solvent
and upon the concentration. Again, no essential distinction can be drawn
between the compounds with complex, metal-containing anions and those which
are formed by the addition of NH 3 , H 2 O, NO 2 , etc., to the metal ion of a simple
salt, and in which the metal-containing cation is complex and the activity of
its constituents is restricted. Finally, certain binary substances, such as for
example the oxides, which themselves possess none of the characteristics of
salts, can produce salts (or acid3 or bases) by uniting with one another. Thus
(^1) It is a quite common practice to indicate complex ions in chemical for-
mulas by enclosing them in brackets.
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