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...
