Page 154 - 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

While these concepts have usually been applied to metal compounds, a w ide range of
other species can be considered to consist of a c entral atom or central atoms to which a
number of other groups are bound. The application of additive n omenclature to such species
is briefly described and exemplified in Chapter I R-7, and abundantly exemplified for
inorganic a cids in Chapter I R-8.

IR-9.1.2.2 Coordination compounds and the coordination entity
A c oordination compound is any compound that contains a c oordination entity. A c oordi-
nation entity is an ion or neutral molecule that is composed of a c entral atom, usually that of
a m etal, to which is attached a s urrounding array of other atoms or groups of atoms, each
of which is called a l igand. Classically, a l igand was said to satisfy either a s econdary or a
primary valence of the central atom and the sum of these valencies (often equal to the
number of ligands) w as called the coordination number (see Section IR-9.1.2.6). In
formulae, the coordination entity i s e nclosed in square b rackets whether it is charged or
uncharged (see Section IR-9.2.3.2).

Examples:

1. [Co(NH 3 ) 6 ] 3þ
2. [PtCl 4 ] 2
3. [Fe 3 (CO) 12 ]

IR-9.1.2.3 Central atom

The central atom is the atom in a c oordination entity which binds other a toms or groups
of atoms (ligands) to itself, thereby occupying a c entral position in the coordination entity.
The central atoms in [NiCl 2 (H 2 O) 4 ], [Co(NH 3 ) 6 ] 3þ and [PtCl 4 ] 2 are nickel, cobalt and
platinum, r espectively. In general, a name f or a ( complicated) coordination entity will be
more easily produced if more central atoms are chosen ( see Section IR-9.2.5)
and the connectivity of the structure is indicated using the kappa convention (see Section
IR-9.2.4.2).

IR-9.1.2.4 Ligands

The ligands are the atoms or groups of atoms bound to the central atom. The root of the word
is often converted i nto other forms, such as to ligate, meaning t o c oordinate as a l igand, and
the derived participles, l igating and ligated. The terms ‘ligating atom’ and ‘donor atom’ are
used interchangeably.

IR-9.1.2.5 Coordination polyhedron
It is standard p ractice to regard the ligand atoms directly attached to the central atom as
defining a c oordination polyhedron ( or polygon) about the central atom. Thus [Co(NH 3 ) 6 ] 3þ
is an octahedral ion and [PtCl 4 ] 2 is a s quare p lanar ion. In such cases, the coordination
number will be equal to the number of vertices in the coordination polyhedron. T his m ay not
hold true in cases w here one or more l igands coordinate to the central atom through two or
more contiguous atoms. It may hold if the contiguous atoms are treated as a s ingle ligand
occupying one vertex of the coordination polyhedron.

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