PAPER www.rsc.org/dalton | Dalton Transactions
Stereochemical nomenclature for octahedral coordination compounds
containing polydentate ligands: a comprehensive proposal
Santiago Herrero*
a
and Miguel A. Us ´ on†
b
Received 25th April 2008, Accepted 7th July 2008
First published as an Advance Article on the web 24th July 2008
DOI: 10.1039/b806050a
A universal nomenclature for the stereoisomers of octahedral coordination compounds containing
polydentate ligands is hitherto lacking. A proposal for naming such compounds in a systematic and
unequivocal way is described. Two skew chelate rings are selected following simple rules, and their
helicity is assigned to the complex. Thence, a single descriptor is obtained and the exhaustive
examination of all the possible pairs of lines is avoided. Another proposal, closer to the current
notation, is reported and the advantages and disadvantages of both are discussed.
Introduction
Since the early development of modern coordination theory in
1893, stereochemistry has played a fundamental rˆ ole. Indeed,
the success of Werner in establishing his co-ordination theory
was based
1,2
on his ability to resolve the enantiomers of cis-
[Co(en)
2
(NH
3
)X]
2+
(X = Cl, Br; en = ethane-1,2-diamine) and
[Co(en)
3
]
3+
.
According to Hawkins,
3
there are six different ways in which an
octahedral coordination compound can attain chirality. Amongst
them, the most studied ones have been: (a) the distribution of
chelate rings about the central metal ion (helicity), and (b) the
distribution of unidentate ligands (configuration) around the
metal centre.
As a consequence of the special increase in stability due
to chelate ligands (chelate effect), early studies focused on
tris(bidentate) and cis-bis(bidentate) complexes. After several
different proposals for the nomenclature of these systems, the D/K
notation was forwarded
4
in 1970 and endorsed
5
by IUPAC in 1971.
As stated by the Commission,
4
“It is desired, in all generality,
to have a designation of chirality which is independent of the
chemical nature of the chelate ligands and which only depends
on the relative positions of [...] the bidentate ligands or the
bidentate units of polydentate ligands”.
For those coordination compounds where the distribution of
donor atoms around the coordination centre is the origin of
chirality, Chemical Abstracts Service introduced
6
in 1972 a unified
system of stereochemical notation, based on the Cahn–Ingold–
Prelog (CIP) standard sequence rule.
7
Although for octahedral
complexes of the tris(chelate) or cis-bis(chelate) type “the chirality
symbols C and A could also be used by observing the priming
principles given in priming subrule a”, the D/K convention was
retained.
6
It must be noted that the application of the priming rule,
for instance, for complexes of the type fac-[M(AB)
3
](i.e.(OC-
6-22)-[M(AB)
3
], see Fig. 1) lead to two different stereochemical
a
Departamento de Qu´ ımica Inorg´ anica, Facultad de Ciencias Qu´ ımicas,
Universidad Complutense, 28040, Madrid, Spain. E-mail: sherrero@quim.
ucm.es; Fax: +34 91 394 4352; Tel: +34 91 394 4344
b
Departamento de Qu´ ımica Inorg´ anica, Facultad de Ciencias, Universidad
de Zaragoza, 50009, Zaragoza, Spain
† Deceased September 26, 2007
Fig. 1 Possibilities after applying the priming convention for
fac-[M(AB)
3
].
descriptors with opposite chirality symbols for a single stereoiso-
mer, depending on which chelate is primed (or doubly primed).
A single descriptor is obtained (C) applying the principle of
trans maximum difference. However, the user must be aware in
advance of the existence of several possibilities and check every
one of them.
Anyway, it is important to notice the different source of
dissymmetry represented by the C/A descriptors (configuration
of donor atoms) from that represented by the D/K convention
(distribution of chelate rings) around the metal centre. In the words
of T. E. Sloan
8
(of Chemical Abstracts Service): “Three different
chirality lables (sic) are employed to designate the chirality at a
coordination center. Although these symbols all convey essentially
the same information (R, C, and D indicating right-handed or
clockwise and S, A, and K indicating left-handed or anticlockwise),
the principles defining right- and left-handed are different for each
set of symbols. Because of the differing assignment principles,
there is no exact translation of R to C or C to D. For this reason the
three different sets of chirality symbols are retained to emphasize
the different set of defining principles”.
Another question is the topology of the compounds. Thus,
for the stereoisomer represented in Fig. 2 of the compound
(N,N
-ethane-1,2-diamine)[[N,N
-ethane-1,2-diylbis(N-methylgly-
cinato)](2−)-N,N
,O,O
]cobalt(1+) the stereochemical descriptor
(OC-6-1
3-A) was proposed.
6
However, the diphosphine derivative shown in Fig. 3, (ethane-
1,2-diphosphane-P,P
)[[ N,N
-ethane-1,2-diylbis(N -methylglyci-
nato)](2−)-N,N
,O,O
]cobalt(1+) (which, not taking into account
the chemical nature of the chelate ligands, is isomorphous with
This journal is © The Royal Society of Chemistry 2008 Dalton Trans., 2008, 4993–4998 | 4993