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

758 Coordination Compounds

TABLE X Diastereoisomer Salt Formation
Racemate (X) Resolving agent (A) Solvent LSD a
(NH 4 ) 2 [Pt(S 5 ) 3 ] (−)-[Ru(bipy) 3 ](ClO 4 ) 2 Acetone (−)A(−)X
[Co(EDTA)] − L-Histidinium Water–ethanol (+)A(+)X
H
N
Na[Rh( SO 2 ) 2 (H 2 O) 2 ] (+)-H 3 CCH(C 6 H 5 )NH 2 Water (+)A(+)X
N
H
[Ru(bipy) 3 ] 3+ (+)-[Co(LCysu) 3 ] 3− Water (+) 350 X(+)A
[Co(en) 2 (NO 2 ) 2 ] + (+)-[Co(EDTA)] − Water (−)X(+)A
a This column describes the less soluble diastereoisomeric (LSD) slat formed from the
resolving agent (A) and one hand of the racemate. It gives (sign of rotation of the cation) (sign
of rotation of the anion). This measurement was done at the yellow lines of sodium, except for
the [Rh(biby) 3 ] 3+ ion, where the rotation is at 350 nm, as indicated by the subscript (+) 350 .

the racemic mixture and right–left for the left-handed half. Of the several claimed separations of D-47 from L-47 (op-
Table X shows some examples of resolutions. tical resolution) at least that of [Co(NH 3 ) 2 (NO 2 ) 2 (CN) 2 ]
Pasteur found for organic racemates involving asym- seems authentic.
metric tetrahedral carbon atoms that microorganisms Quite apart from such modification of essentially sym-
(themselves made up of chiral molecules all of one hand; metric shapes, there is another general effect, causing dis-
i.e., bacteria are very stereospecific reagents) metabolize tortion of symmetric shapes, that is particularly common
one hand much more rapidly than the other. In much among coordination compounds of some transition metals
the same way, microorganisms show at least stereos- (but also known for other species, such as the first elec-
electivity and occasionally, apparently, stereospecificity tronic excited state of benzene). This is the Jahn–Teller
(100% selection) for those few octahedral chelated com- effect. For nonlinear assemblies of nuclei and electrons,
pounds that have been studied. For example, racemic unequal occupancy of degenerate orbitals is unstable, and
mer-trisglycinatocobalt(III) serves as a nitrogen source there will be a lower energy state of different geometry in
for Pseudomonas stutzeri, but only the D hand is con- which the nuclear framework has been distorted. Taking
4
sumed; the L is unaffected. This is the best way to ob- the electronic configurations of spin-free d [e.g., Cr(II),
9
tain this uncharged complex compound in optically active Mn(III)] and any d [e.g., Cu(II), Ag(II)] as examples, in
form. octahedral ligand fields the electron occupancy is shown
in 48 and 49,
G. Jahn–Teller Effect
For the common four- and six-coordinated shapes, lower-
ing of symmetry by di- or trisubstitution of one ligand by
another leads to geometric isomerism (where the shape of
the field around the metal ion differs between the two iso-
mers, e.g., fac and mer). Similarly, for the six-coordinated
octahedron (O h ), lowering of symmetry by removing re-
flection elements (center, plane, or improper axis), as in respectively. The Jahn–Teller theorem says that the nu-
47, gives the clei will move to lift the degeneracy (i.e., x = y = z), as in
Fig. 6. There are many cases of such distortion. Indeed,
either (or both) the ground state or the first excited state of
n
all configurations d (1 ≤ n ≤ 9) is subject to Jahn–Teller
distortion. Particularly noticeable are departures from reg-
ular shape in which the unequally occupied degenerate
orbitals are antibonding; some examples are given in
Table XI. The energy level diagram for T d for a partic-
n
n
possibility of enantiomerism. This is usually done by ular d is the inverse of that for O h , for the same d shown
chelating. Examples of racemates D-47 + L-47 are known. in Figs. 4 and 5.