Page 182 - Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005)

IR-9.2 C O O R D I N A T I O N C O M P O U N D S

Table IR-9.1 Structural descriptors

Number of Descriptor Point group CEP descriptor
atoms in CSU

3 triangulo D 3h
4 quadro D 4h
4
4 tetrahedro T d [T d -(13)-D -closo]
6
5 D 3h [D 3h -(131)-D -closo]
8
6 octahedro O h [O h -(141)-D -closo]
6 triprismo D 3h
8 antiprismo S 6
6
8 dodecahedro D 2d [D 2d -(2222)-D -closo]
8 hexahedro (cube) O h
20
12 icosahedro I h [I h -(1551)-D -closo]

In brief, the numbering of the CSU is based on locating a r eference axis and planes of
atoms perpendicular t o t he reference axis. The reference axis is the axis of highest
rotational s ymmetry. S elect that end of the reference axis with a s ingle atom (or smallest
number of atoms) in the first plane t o b e n umbered. Orient the CSU so that the first
position to receive a l ocant in the first plane with more t han one atom is in the twelve
o’clock position. Assign locant n umbers to the axial p osition or to each position in the
first plane, beginning at the twelve o’clock position and moving in either the clockwise or
anticlockwise direction. From the first plane m ove to the next position and continue
numbering in the same d irection (clockwise or anticlockwise), always returning t o t he
twelve o’clock position or the position nearest to it in the forward d irection before
assigning locants in that plane. Continue numbering in this manner until all positions
are numbered.
8
A f ull discussion of numbering deltahedra may be found elsewhere. The complete
descriptor for the CSU should appear j ust before t he central atom list. Where structurally
significant, metal–metal bonds may be indicated (see Section IR-9.2.5.3 and examples
below).
The chain o r r ing s tructure numbering in a C SU must b e c onsecutive and only
thereafter o bey rules (a)–(d) given i n S ection IR 9.2.5.6. In Example 3 b elow, the CSU
numbering in fact coincides with the numbering that would be reached using those rules
alone.

Examples:
1. [{Co(CO) 3 } 3 (m 3 -CBr)]
(m 3 -bromomethanetriido)nonacarbonyl-triangulo-tricobalt(3 Co—Co), or
(m 3 -bromomethanetriido)-triangulo-tris(tricarbonylcobalt)(3 Co—Co)

2. [Cu 4 (m 3 -I) 4 (PEt 3 ) 4 ]
tetra-m 3 -iodido-tetrakis(triethylphosphane)-tetrahedro-tetracopper, or
4
tetra-m 3 -iodido-tetrakis(triethylphosphane)-[T d -(13)-D -closo]-tetracopper

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