Page 156 - Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005)

IR-9.1 C O O R D I N A T I O N C O M P O U N D S

If a p otentially bidentate ligand, such as ethane-1,2-diamine, c oordinates to two metal
ions, it does not chelate but coordinates in a m onodentate fashion to each metal ion, forming
a c onnecting link or bridge.

Example:

1. [(H 3 N) 5 Co(m-NH 2 CH 2 CH 2 NH 2 )Co(NH 3 ) 5 ] 6þ

Alkenes, arenes and other unsaturated m olecules attach to central atoms, using some o r a ll
of their multiply bonded atoms, to give organometallic complexes. While there are many
similarities between t he nomenclature of coordination and organometallic compounds,
the latter differ from the former in clearly definable ways. Organometallic complexes are
therefore treated separately i n C hapter IR-10.

IR-9.1.2.8 Oxidation s tate

The oxidation state of a c entral atom in a c oordination entity is defined as the charge it
would bear if all the ligands were r emoved along w ith the electron pairs that were shared
with the central atom. It is represented by a R oman numeral. It must be emphasized that
oxidation state is an index derived from a s imple and formal set of rules (see also Sections
IR-4.6.1 and IR-5.4.2.2) and that it is not a d irect indicator of electron distribution. I n c ertain
cases, the formalism does not give acceptable central atom oxidation states. Because o f s uch
ambiguous cases, the net charge on the coordination entity is preferred in most n omenclature
practices. T he following examples illustrate the relationship between t he overall charge on a
coordination entity, the number and charges of ligands, and the derived central atom
oxidation state.

Formula Ligands Central atom
oxidation state
1. [Co(NH 3 ) 6 ] 3þ 6 N H 3 III
2. [CoCl 4 ] 2 4 C l II
3. [MnO 4 ] 4 O 2 VII
4. [MnFO 3 ] 3 O þ1 F VII
2
5. [Co(CN) 5 H] 3 5 C N þ1 H III

6. [Fe(CO) 4 ] 2 4 C O II

IR-9.1.2.9 Coordination nomenclature: an additive nomenclature

When coordination theory was first developed, c oordination compounds were considered
to be formed by addition of independently stable compounds to a s imple central compound.
They were therefore named on the basis o f a n a dditive principle, where the names of the
added compounds and the central simple compound were combined. This principle remains
the basis for naming coordination compounds.
The name is built up around t he central atom name, just as the coordination entity is built
up around the central atom.

147