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

Coordination Compounds 741

penicillamine (3) or triethylenetetramine (4). The latter The position marked with an asterisk across from A in 6
is systematically named 1,4,7,11-tetraazaundecane. (trans to A) is unique. The replacement of a second water
in 6 by ammonia gives the disubstituted complex; there
are in 6 four positions next (cis) to A, which gives 7, and
only one across from A (trans), which gives 8:

If a third W is replaced in 7 or 8 by A, the trans (8, 20%
of the whole) can give only 9, but the 80% that was cis
(7), gives 40% mer (9) and 40% fac (10) for a final result
of 60% mer-9 and 40% fac-10.

II. FORMATION

A. Equilibria between Cations
and Donors in Water
When a donor molecule (a Br¨onsted base, i.e., a proton
acceptor) forms a bond by donating electron density to a
A fourth replacement gives, from the 40% fac (10),
positive center (a Lewis acid, i.e., an electron acceptor),
only cis-A 4 W 2 (since all three Ws are equivalent), but
coordination is said to occur, as shown in Eq. (4).
from mer, the position marked with an asterisk is unique,
AlCl 3 + Cl → [AlCl 4 ] − (4) giving trans-W 2 A 4 for one-third of the replacements into
−
More commonly, coordination occurs in solution (often mer (A 3 W 3 ) (i.e., 20% of the total, since mer was 60%
aqueous), to give overall processes of metal complexing of the whole). Thus, we have 20% trans-[MA 4 W 2 ] and
such as 80% cis-[MA 4 W 2 ]. The fifth substitution, of course, gives
MA 5 W.
Ni 2+ (aq) + 6NH 3 → Ni(NH 3 ) 2+ (aq) (5)
6 There is an obvious symmetry to this series of replace-
In fact, the six water ligands (W) are being replaced, one ments: We may start from MW 6 and replace W by A, or
by one, stepwise, by six ammonia ligands: vice versa [Eq. (6)]:

MA 6 + W MA 5 W 1 MA 4 W 2 MA 3 W 3

MA 2 W 4 MA 1 W 5 MW 6 + A (6)

Notice that six-coordinate complex ions [MA n B 6−n ],
where 2 ≤ n ≤ 4, may exist in two forms. In the ex-
ample above, A is NH 3 and B is H 2 O, but the occurrence
of two forms (7 as against 8,or 9 as against 10) differing
The replacement of one water in 5 by ammonia (A) gives
in the arrangement of ligands A and B about the center is
the monosubstituted complex 6:
quite general.

B. Stability Constants and Relationships
among Them
The general form (7) of the overall stability constant for
the coordination