Nomenclature of Inorganic Chemistry II

By J.A. McCleverty, N.G. Connelly

The Royal Society of Chemistry

Copyright © 2001 International Union of Pure and Applied Chemistry
All rights reserved.
ISBN: 978-0-85404-487-0

Contents

Principal authors of this Edition, vii,
Preface, viii,
II-1, POLYANIONS]TP1 TP1[1,
II-2, ISOTOPICALLY MODIFIED INORGANIC COMPOUNDS]TP1 TP1[23,
II-3, METAL COMPLEXES OF TETRAPYRROLES]TP1 TP1[36,
II-4, HYDRIDES OF NITROGEN AND DERIVED CATIONS, ANIONS AND LIGANDS]TP1 TP1[54,
II-5, INORGANIC CHAIN AND RING COMPOUNDS]TP1 TP1[62,
II-6, GRAPHITE INTERCALATION COMPOUNDS]TP1 TP1[95,
II-7, REGULAR SINGLE-STRAND AND QUASI SINGLE-STRAND INORGANIC AND COORDINATION POLYMERS]TP1 TP1[104,
INDEX, 126,

CHAPTER 1

Polyanions

CONTENTS

II-1.1 Introduction
II-1.2 Numbering of condensed polyanions
II-1.2.1 Choice of reference axis
II-1.2.2 Choice of preferred terminal skeletal plane
II-1.2.3 Choice of reference symmetry plane
II-1.2.4 Numbering central atoms
II-1.2.5 Octahedron vertex designation
II-1.3 Polyanions with six central atoms
II-1.3.1 Homopolyanions (isopolyanions)
II-1.3.2 Heterocentre polyanions
II-1.3.2.1 Mono- or polysubstitution
II-1.3.2.2 Reduced heterocentre polyanions
II-1.3.3 Heteroligand polyanions
II-1.3.3.1 Single substitution
II-1.3.3.2 Several substitutions
II-1.3.4 Names of more complicated species
II-1.4 Polyanions with the Anderson structure
II-1.4.1 Polyanions with seven central atoms
II-1.4.2 Names of more complicated species
II-1.5 Polyanions with twelve central atoms
II-1.5.1 Compounds with the Keggin structure and isomers
II-1.5.1.1 Compounds containing only one kind of transition metal
II-1.5.1.2 Compounds with several transition metals, i.e. substituted compounds
II-1.5.1.3 Ligand substitution
II-1.5.1.4 Reduced compounds
II-1.5.2 Compounds in which central atoms are missing (defect structures)
III.5.2.1 Compounds with one vacancy
II-1.5.2.2 Compounds with three vacancies
II-1.6 Polyanions with eighteen central atoms
II-1.7 Conclusion

II-1.1 INTRODUCTION

A polyanion is formed by the condensation of several simple anions with the elimination of water. These negatively charged species have structures mainly made up of octahedra (polytungstates or polymolybdates), tetrahedra (polyphosphates), and sometimes octahedra and tetrahedra (polytungstates or polymolybdates). The octahedra and tetrahedra consist of a central atom surrounded by six or four atoms, respectively, which are referred to as ligands in this Chapter. The octahedra and tetrahedra share edges and vertices. The structure considered as an unsubstituted parent is the one which contains oxygen as ligands. Central atoms may be atoms of metals or, sometimes, non-metals. Some rare cases of 5-atom coordination and 7-atom coordination are known.

Either a central atom or a ligand can be replaced. Therefore, every atomic position must be numbered in order to be recognized and to distinguish isomers. In the nomenclature of coordination compounds, lower case letters have been suggested as locant designators for vertex designation. Central atoms have not commonly been given locant designators; how- ever, number locants have been used for numbering metal atoms in homoatomic aggregates. In the first case, the position of a ligating atom of the ligands in the coordination polyhedra is given by a lower case letter. In the latter case, the ligand atom is indicated by a number which defines the central atom to which it is bound; if the ligand bridges several central atoms, several numbers are used. Thus, two locant systems presently coexist.

In the specific case of polyanions, difficulties arise because both central atoms and ligands can be replaced. The number of vertices of a condensed species is, in most instances, quite large: for example, [SiW12O40]4- has 40 vertices and is far from being the largest known poly anion. Obviously, the 26 letters of the alphabet are not sufficient if they are used for designating each vertex position. Since it is necessary to distinguish isomers, an unambiguous designation for central atoms, as well as for vertices, has to be devised. Moreover, the use of the numbers of two central atoms is not sufficient for designating bridging atoms because two or more bridges can occur between the same two central atoms.

The following numbering system is proposed:

(a) each central atom is given a number: 1, 2, 3, etc.,

(b) each polyhedron vertex is given a letter:

octahedron – a, b, c, d, e,

tetrahedron – a, b, c, d.

A vertex is then designated by a number followed by a letter, the number referring to the central atom, e.g. 1a, 3d, etc. Thus, when two octahedra share a vertex, this vertex has two designations, one from the first octahedron, and one from the second octahedron, each octahedron surrounding its central atom. The designation with the lowest central atom number takes precedence. For example, if a vertex is 1d in the first octahedron and 4a in the second, it is designated by 1d.4a. Such a multiple designation might appear redundant. However, it may prove distinctly useful: for instance, in a discussion involving ligands located at vertices 1d and 4f, if 4a is an alternative for 1d, then 4a may be used instead of 1d to make it quite obvious that the two vertices, 4a and 4f, belong to the same octahedron. Moreover, this double designation makes it quite simple to name a common vertex: e.g. 1d.4a shows that vertex 1d is also 4a thus bridging central atoms 1 and 4 by their respective vertices d and a.

The numbering system used in this Chapter is consistent with the principles developed for boron cage compounds in Section I-11 of Note 1a and names are based on coordination nomenclature in the same book (Section I-10 of Note 1a), not on traditional oxoanion nomenclature, e.g. tetraoxophosphate(3–), not phosphate.

II-1.2 NUMBERING OF CONDENSED POLYANIONS

The numbering of a condensed structure is based on the unsubstituted parent structure for the polyanion. The central atoms of the octahedral units are numbered and the ligand positions are indicated by a secondary set of letter locants. Tetrahedral units are treated as bridging ligands.

Polyhedra constructed from octahedra contain symmetry axes of rotation and skeletal planes. Such planes are defined as those planes (or quasi-planes) containing several octahedral centres.

The following numbering recommendations are applied sequentially.

II-1.2.1 Choice of reference axis (see Figure II-1.1)

(a) The reference axis is the rotational axis of the polyanion structure of highest order; it is oriented vertically.

(b) Perpendicular to the reference axis, several skeletal planes may be encountered. A skeletal plane which lies farthest from the centroid of the polyanion is described as a terminal skeletal plane, others as internal skeletal planes.

(c) When there is more than one symmetry axis of highest order, the preferred axis is that which is perpendicular to the greatest number of skeletal planes.

(d) When the polyanion has no axis of rotational symmetry, the reference axis is then the axis perpendicular to the skeletal plane with the greatest number of octahedral centres.

II-1.2.2 Choice of preferred terminal skeletal plane

(a) The preferred terminal skeletal plane is that plane with the least number of central atoms. The reference axis is then oriented in such a way that the preferred terminal plane is uppermost.

(b) When both terminal planes contain the same number of central atoms, the preferred plane is that with the least condensed fusion of octahedra (i.e. when the number of bridges between cental atoms is the lowest; vertex sharing is less condensed than edge sharing which is less condensed than face sharing).

(c) See Section II.1.2.4 below.

(d) When a further choice is necessary, the preceding rules are applied considering the first internal skeletal planes, and so on.

II-1.2.3 Choice of reference symmetry plane

(a) The reference plane is defined as the symmetry plane which contains the reference axis and which also contains the lowest number of central atoms.

(b) When there is more than one reference symmetry plane which satisfies this requirement, then the preferred plane is that which contains the most atoms in common with the preferred terminal skeletal plane.

(c) The reference symmetry plane is divided by the reference axis in two halves which must be designated. A 6 o’clock – 12 o’clock line is defined by the intersection of the reference symmetry plane and a skeletal plane; thus it is perpendicular to the reference axis, and the 12 o’clock position is in the half of the reference plane which contains the largest number of central atoms; 6 o’clock designates the other half.

(d) See Sections II-1.2.4 and II-1.2.5 below.

(e) When a choice is left, the 12 o’clock position is chosen on a ligating atom.

II-1.2.4 Numbering central atoms

(a) Central atoms are numbered starting from the 12 o’clock position in the preferred terminal skeletal plane and turning clockwise (or anticlockwise). When a skeletal plane is fully numbered, the next skeletal plane located immediately below is numbered; the first atom to be numbered is that met when starting from the 12 o’clock position, turning clockwise or anti-clockwise, depending on the lowest locant requirement [see Sections II-1.2.4(b) and II-1.2.5].

(b) When a central atom or a ligand is substituted (see Section II-1.2.5), it does not lower the symmetry of the skeleton for the choice of the reference axis of symmetry and for the choice of the reference symmetry plane. However, locants are chosen in such a way that a central atom or a ligating atom appearing first in Table II-1 has the lowest number or the earliest letter. Nevertheless, the choice of the first skeletal plane as defined in Section II-1.2.2 takes precedence over the number of a substituting central atom. When the polyanion contains several central atoms of several different atomic species, the largest number of atoms of the species coming first in Table II-1 will be numbered before numbering an atom coming second in this Table, and so on.

II-1.2.5 Octahedron vertex designation

(a) In each octahedron letter locants are assigned to vertices as follows: define an axis passing through the central atom of the considered octahedron and parallel to the main reference axis. This defined axis is the local reference axis. These two axes make a new reference plane valid for this octahedron only. The vertices of the octahedron are in local skeletal planes perpendicular to these two axes. In each local skeletal plane, the local 6 o’clock – 12 o’clock line is the line intersecting the local reference axis and the main reference axis. The intersection with the main axis is the 12 o’clock position in the considered local skeletal plane.

Letters a, b, c, d, e, f are assigned starting from the upper local skeletal plane. The (possibly several) vertices in a given local skeletal plane are assigned letters turning clockwise around the local axis, starting from the local 12 o’clock position. If the local and the main axis coincide, then the local reference plane is the polyanion reference plane. The same set of rules is applied sequentially to give a letter locant designator to each vertex.

(b) If a choice exists in assigning letter locants to vertices, vertices are ordered according to the position in Table II-1 of the ligand atoms occupying them. An earlier position in this Table is assigned a letter earlier in the alphabet. In this connection, monoatomic ligands precede polyatomic ligands with the same ligating atom, e.g. oxygen atoms of OH or CH3COO are considered to come immediately after oxygen and before any other element.

II-1.3 POLYANIONS WITH SIX CENTRAL ATOMS

The first representative example of a metal polyanion with six central atoms to have its structure determined was K8[Nb6O19] by Lindqvist. The idealized structure has Oh symmetry (Figure II-1.1). Several modifications of this structure are known:

(a) all central atoms are identical: such ions are commonly called isopolyanions; a better name is homopolyanions;

(b) one or more central atoms are substituted; these ions are commonly called mixed polyanions or heteropolyanions;

(c) some ligands are substituted.

Since substitution can occur either at a central atom or at a ligand site, these substituted ions are named as heterocentre polyanions or heteroligand polyanions, respectively.

II-1.3.1 Homopolyanions (isopolyanions)

In this structure, the metal atom has only oxygen ligands and there are six fused octahedra; it is sufficient to count the number of oxygens of each kind, i.e. of the same coordination.

Examples:

1. [Nb6O19]8-

μ6-oxo-dodeca-μ-oxo-hexaoxohexaniobate(8–)

2. [W6O19]2-

μ6-oxo-dodeca-μ-oxo-hexaoxohexatungstate(2–)

Multiplicative prefixes may be used to provide alternative shorter names, e.g. with equivalent NbO or WO groups.

3. [Nb6O19]8-

μ6-oxo-dodeca-μ-oxo-hexakis(oxoniobate)(8–)

A homopolyanion (isopolyanion) can be reduced without electron localisation. Such compounds are characterized by intervalence charge transfer absorptions in their electronic spectra and are commonly termed ‘mixed valence compounds’. The above names are used with the resulting charge expressed by the charge number. For example, [Mo6O19]2- can be reduced to [Mo6O19]3- which may be named as follows:

Example:

4. [Mo6O19]3-

μ6-oxo-dodeca-μ-oxo-hexakis(oxomolybdate)(3–)

However, when it is necessary to express electron localization in the reduced species, the polyanion is named in the same manner as a heterocentre polyanion. In this case, the atom with lower oxidation state comes first in the formula and precedes those with the higher valencies in the name. When numbering central atoms, the lowest possible number is assigned to the reduced atom.

II-1.3.2 Heterocentre polyanions

II-1.3.2.1 Mono- or polysubstitution

In names, central atoms are cited in alphabetical order independently of the numbering scheme. The list of central atom element names is enclosed in parentheses with the ending -ate after the parenthesis.

Examples:

1. [NbW5O19]3-

μ6-oxo-dodeca-μ-oxo-hexaoxo(niobiumpentatungsten)ate(3–)

2. [Nb5WO19]7-

μ6-oxo-dodeca-μ-oxo-hexaoxo(pentaniobium-1tungsten)ate(7–), or μ6-oxo-dodeca-μ-oxo-hexaoxo[pentaniobium(v)-1-tungsten(vi)]ate Tungsten is assigned number 1 because of its position in Table II.1.

3. [Nb4W2O19]6-

μ6-oxo-dodeca-μ-oxo-hexaoxo(tetraniobium-1,2-ditungsten)ate(6–), or μ6-oxo-dodeca-μ-oxo-hexaoxo(tetraniobium-1,6-ditungsten)ate(6–)

In this example, two isomers occur; they are commonly designated cis and trans. The numbering system outlined above coupled with coordination nomenclature provides a unique name for each isomer.

4. [Nb4W2O19]6-

μ6-oxo-dodeca-μ-oxo-hexaoxo(tetraniobiumditungsten)ate(6–)

When the substituted positions are not known, locant designators are not given.

5. [V2W4O19]4-

μ6-oxo-dodeca-μ-oxo-hexaoxo(tetratungsten-5,6-divanadium)ate(4–), or μ6-oxo-dodeca-μ-oxo-hexaoxo(tetratungsten-3,5-divanadium)ate(4–) Vanadium locants are related to the vanadium position in Table II.1

6. [NbVW4O19]4-

μ6-oxo-dodeca-μ-oxo-hexaoxo(5-niobiumtetratungsten-3-vanadium)ate(4–)

II-1.3.2.2 Reduced heterocentre polyanions

Generally, the most easily reducible central atom is known.

(Continues…)Excerpted from Nomenclature of Inorganic Chemistry II by J.A. McCleverty, N.G. Connelly. Copyright © 2001 International Union of Pure and Applied Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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