Elemental high coordination numbers

The fluoride ion is the least polarizable anion. It is small, having a diameter of 0.136 nm, 0.045 nm smaller than the chloride ion. The isoelectronic E and ions are the only anions of comparable size to many cations. These anions are about the same size as K" and Ba " and smaller than Rb" and Cs". The small size of E allows for high coordination numbers and leads to different crystal forms and solubiUties, and higher bond energies than are evidenced by the other haUdes. Bonds between fluorine and other elements are strong whereas the fluorine—fluorine bond is much weaker, 158.8 kj/mol (37.95 kcal/mol), than the chlorine—chlorine bond which is 242.58 kJ/mol (57.98 kcal/mol). This bond weakness relative to the second-row elements is also seen ia 0-0 and N—N single bonds and results from electronic repulsion. 

Table 26.2 also reveals a diminished tendency on the part of these elements to form compounds of high coordination number when compared with the iron group and, apart from [Co(N03)4], a coordination number of 6 is rarely exceeded. There is also a marked reluctance to form oxoanions (p. 1118). This is presumably because their formation requires the donation of n electrons from the oxygen atoms to the metal and the metals become progressively... 

High coordination numbers also exist with amido ligands. However, because of the steric requirements of even the smaller ligands, such as NMe2, examples of five- and six-coordination are encountered only with the heavier elements. 

Typical metallic substances have structures which cannot readily be described either in terms of directional covalent bonds or as arrays of cations and anions. Metallic elemental substances exhibit high coordination numbers, and can - to a first approximation - be viewed as arrays of cations embedded in a sea or glue of electrons completely delocalised over the crystal. This model helps to explain the characteristic mechanical, thermal and electrical properties of metals. It is also consistent with... 

The majority of elemental substances, and a large number of compounds, have metallic properties (see Section 3.3). Metallic elemental substances are characterised by three-dimensional structures with high coordination numbers. For example, Na(s) has a body-centred cubic (bcc) structure in which each atom is surrounded by eight others at the corners of a cube, each at a distance of 371.6pm from the atom at the centre. The Na atom also has six next-nearest neighbours in the form of an octahedron, with Na-Na distances of 429.1pm. A fragment of this lattice is shown in Fig. 7.14. These distances may be compared with the Na-Na bond length of 307.6 pm in the Na2 molecule, which can be studied in the gas phase by vaporisation of sodium metal. 

The covalent radii of transition elements are subject to two additional effects that influence the values of ionic radii also. A large covalent radius for a given atom is favored by both a low oxidation number and a high coordination number. These two effects are independent neither of each other nor of bond order effects however, an adequate unified treatment of the interrelationships between bond number, coordination number, oxidation number, and bond distances for compounds of the transition metals is best postponed to a more advanced text. 

Most elements (including the /-groups) in the oxidation state z have the property (82) that I of the gaseous or / of the solid fluoride is approximately z eV higher than of the corresponding oxide. This difference is far smaller in the immediate post-transition elements such as Zn(II), Cd(II) and Hg(II). This fact may conceivably be connected with the usually high coordination number N = 6,8 and 8 of these elements. 

Platinum is found in compounds having a smaller range of oxidation states than is the case for many of the earlier elements in the periodic table. The differences between it and Pd are somewhat more marked than for analogous pairs of earlier elements. The coordination numbers see Coordination Numbers Geometries) tend to be lower than for earlier elements a CN of six is rarely exceeded and a CN of four is common. Many important concepts in coordination chemistry, such as square-planar coordination and the trans effect, were first discovered in Pt complexes. The high electronegativity of the element is reflected in a poor 7t-basic character, which helps account for the lack of a binary carbonyl. 

The results of solid state reactions of protactinium dioxide and pentoxide with other metal oxides (89, 93-96) support the view that the oxide systems of protactinium resemble those of other actinide elements rather than those of niobium and tantalum. However, when assessing results of this type one must always bear in mind the relative ionic radii of the respective M " and M + ions since they obviously play a large part in determining the structures of the complex phases. This comment applies equally well, of course, to the structural properties of other types of compound and in particular to the high coordination numbers exhibited by protactinium(V) in its chloro and nitrato complexes. 

Bi complexes are summarized in Table 5. The lack of As(V) complexes of the type R3 AsL2 may be explained by the resistance of As to realize high coordination numbers and oxidation states, which are usually discussed on the basis of the post-transition metal effect. Element(V) species of the type R3EL possessing only one carboxylato chelate ligand which coordinates symmetrically show trigonal bipyramidal structures. 

The tendency of a metal ion to form compounds of high coordination numbers decreases across the first row of the transition elements. Explain. 

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