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756 Coordination Compounds

other coordination numbers. The extraordinarily selective D. Optical Activity
synthesis form the dianion of the natural amino acid L-
Finally, there are for the fac isomer 37 (point group C 3 )
cysteine as a bidentate ligand at cobalt(III), to give dark
(and indeed the mer isomer 38, C 1 , or for any other ge-
green tris-L-cysteinato-(N,S)-cobaltate(III), illustrates the
ometry that has only axial symmetry, D 3 , C 2 , and so
chief causes of isomerism among coordination com-
on) two forms of the molecule that differ, as do right-
pounds.
and left-handed propellers (39 and 40). Such an object
L-Cysteinate (the doubly charged anion) is represented
(molecule) cannot be superimposed on its image in a mir-
by structure 36.
ror. The labels attached (by arbitrary convention) to pro-
peller molecules, such as

The three atoms underlined are all possible donor sites,
but since only two are used to form each of the three
chelate rings, this ligand could be attched through (1) ni- are shown [from minus (M), plus (P), sinistral (S), rectal
trogen and oxygen (chelated α-amino acidate, with a pen- (R); refers to left-helicity about the threefold axis,
to
dant unattached thiolate, –CH 2 S ), (2) sulfur and oxy- right; D and L are for dextro (right) and levo (left)].
−
gen (a chelated β-thiocarboxylate, with unattached NH 2 ),
or (3) sulfur and nitrogen (chelated 1-amino-2-thiolate,
with pendant carboxylate). This variability of mode of at-
E. Stereoselectivity
tachment of a ligand is called linkage isomerism. Other
ligands with such multiplicity of possible binding sites To distinguish right from left requires an all-one-handed
may be unidentate and include (among several) nitrite at- agent (either all right or all left). For an all-right-disting-
tached via nitrogen, for example, [Co(NH 3 ) 5 (NO 2 )], so- uishing agent, the differing interaction combinations are
called nitropentamminecobalt(III) ion, a yellow ion; ni- right–right and right–left. This is the same difference
2+
trite attached via oxygen, [Co(NH 3 ) 5 (ONO)] , so-called as between shaking hands and holding hands. Isomers
nitritopentamminecobalt(III) ion, a red ion; thiocyanate formed from differing combinations of right- and left-
attached via sulfur, for example, [Rh(NH 3 ) 5 (SCN)] ;or handed (chiral) parts of molecules are known as di-
2+
2+
“isothiocyanate,” nitrogen-bonded [Rh(NH 3 ) 5 (NCS)] . astereoisomers. Natural (optically active) L-cysteine se-
Representing the bidentate L-cysteinate as N–S, three lects the S(C 3 ) propeller of cobalt(III). Such selection
of these may be coordinated as in 37 or 38. among the posible diastereoisomers is known as stere-
oselectivity.
The sole product, from among all the possible iso-
mers, is S(C 3 )-fac-[Co(L-cysteinate-N,S) 3 ] , which can
3−
be readily crystallized as its potassium salt. Such selective
syntheses (where one among several possible reactions oc-
curs preferentially rather than at random) are now more
readily achieved than hitherto.
Tartaric acid (or its conjugate bases) can select be-
tween the right and left hands (41 and 42)of DL-
+
In the discussion of stepwise stability constants [see [Co(en) 2 (CO 3 )] , giving the very stable diastereoisomer
Eq. (5)], the more symmetric facial and less symmet- 43. This effectively takes all the L-cobalt centers out of
ric meridional isomers of [MA 3 B 3 ] were distinguished commission, so that on adding symmertric reagents (e.g.,
−
from their probabilities of formation (mer-9, 60%; fac-10, NO ), the only place for reactions is the D-cobalt of 44,
2
40%). In the present case, only the fac isomer is formed. giving the D-[Co(en) 2 (NO 2 ) 2 ] ion 45a. The overall ionic
+
Such geometric isomers have the same composition but charges on the complex ions 41–46 are omitted, but all the
different molecular symmetries. complexed cobalt centers are cobalt(III).