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Coordination Compounds 759

cobalt(III), with six ligands, five of which are imine-like
or heterocyclic nitrogen, and the sixth an alkyl group (or
in the well-known artifact, now called cyanocobalamin,
formed during the original isolations, cyanide).
The striking fact about biological metal complex chem-
istry is its novelty. When first isolated, metal-containing
molecules and metal-in-volving systems from living cells
almost always turn out to have features that have no real
counterpart in known synthetic (in vitro) coordination
compounds. Much effort has been expended on devel-
oping “model systems” to reproduce these natural bio-
chemical ingenuities. For example, the stable carbon–
FIGURE 6 Two possible distortions (along normal coordinates) cobalt(III) bond in vitamin B 12 has been mimicked in
of an octahedral compound MX 6 with all six MX distances equal the bis-1,2-dionedioximatocobalt(III) moiety (51) (the so-
s
1
(center point) to “four short, two long” MX X or “two short, four
4 2 called cobaloximes), in which the nitrogen donors of the
2 1
long” MX X .
s 4 oxime ligands (whose imine function is very similar to that
in N-heterocycles) are held planar by strong intramolecu-
lar hydrogen bonds.
V. BIOLOGICAL METAL COMPLEXING
The number of metals known to be essential to a range of
living species has increased markedly during the twenti-
eth century, so that the “biological periodic table” is now a
large fraction of the periodic table itself. Most of the metal
requirements are as trace elements, such as vanadium in
mushrooms. The vanadium (in the famous Amanita mus-
caria, the red-capped mushroom with white spots) occurs
in the same coordination compound throughout the mush-
room. This compound, amavanadin, originally isolated
from a Black Forest species, is said to have structure 50.

One reminder of the power of natural syntheses is the
range of iron-binding molecules that have been found
in bacteria. Several functional groups have emerged by
human design as useful ligands to form stable complex
Many natural coordination compounds exist—for ex-
compounds in aqueous media specifically with iron(II) or
ample, vitamin B 12 , in which the central metal is
iron(III). These are shown in Table XII. How wonderful
that the same groups are used by all living things. For
TABLE XI Jahn–Teller Distorted Structures example, the diimine molecules 52, caerulomycin A, and
53 (ferropyrimine) of Table VII are very closely akin to
General shape Configuration Ion Example

the 2,2 -bipyridyl compounds so common in analytical
O h , antibonding (3d) 4 Mn(III) MnF 3 chemistry.
(d 2 −y 2,d 2)
z
x
(3d) 4 Cr(II) KCrF 3
(3d) 9 Cr(II) K 2 CuF 4 VI. THERAPEUTIC METAL COMPLEXES
T d , antibonding (3d) 8 — NiCr 2 O 4
(d xy ,d yz ,d zx )
Whetherasolutionofacoordinationcompoundisingested
(3d) 8 — Cs 2 [CuCl 4 ]
or injected, it enters an aqueous medium and, depending