Binary fluorides of the transition metals

SOLID-STATE STRUCTURES OF THE BINARY FLUORIDES OF THE TRANSITION METALS... 

Solid-State Structures of the Binary Fluorides of the Transition Metals A. J. Edwards... 

The atomic and ionic properties of an element, particularly IE, ionic radius and electronegativity, underly its chemical behaviour and determine the types of compound it can form. The simplest type of compound an element can form is a binary compound, one in which it is combined with only one other element. The transition elements form binary compounds with a wide variety of non-metals, and the stoichiometries of these compounds will depend upon the thermodynamics of the compound-forming process. Binary oxides, fluorides and chlorides of the transition elements reveal the oxidation states available to them and, to some extent, reflect trends in IE values. However, the lEs of the transition elements are by no means the only contributors to the thermodynamics of compound formation. Other factors such as lattice enthalpy and the extent of covalency in bonding are important. In this chapter some examples of binary transition element compounds will be used to reveal the factors which determine the stoichiometry of compounds. 

Structural information on transition-metal binary fluorides - shows that from group V through to the end of each transition series the transition-metal atom is coordinated by six fluorine ligands (F) on an octahedral framework. This is so for hexafluorides, pentafluorides, tetrafluorides, or trifluorides. 

An extremely important class of fluoro-anions will be presented in Sec. 11.3.5 where it will be shown that it is possible to generate fluoro-anions of transition metals in unusually high oxidation states, e.g. MnIVF, AgmF4 and NilvFj and to isolate the corresponding binary fluorides from HF solutions of these anions. 

Zemva, Bartlett and colleagues reported a general approach to synthesis from HF of polymeric binary fluorides [67] which has a major advantage over many of the synthetic routes used to date. Some of these fluorides are thermodynamically unstable or marginally stable at ambient temperature. Earlier attempts to prepare some of these compounds at elevated temperatures had proved unsuccessful or had led to contaminated products. Some reported synthetic procedures for transition metal tetrafluorides, for example, involving reduction by the metal of the stable pentafluoride at elevated temperature have resulted in products contaminated to greater or lesser extent with the trifluoride or with the metal. 

The syntheses cited in this section have all involved attainment of high oxidation states in transition metal fluoro-anions, and thence in binary fluorides and in cationic species, by oxidation with F2 or other fluoro-oxidants in HF of metals or of compounds in lower oxidation states. A couple of examples are offered of syntheses of new fluoro-oxo-compounds involving fluorination of oxides already in high oxidation states with rare gas fluorides in HF. The ratio of F 0 in the ligands of these new compounds is greater than in the related compounds already reported. 

Unfortunately, much of the other available data " on the transition-element binary fluorides is so imprecise that the generality of the unexpected slight decrease in M-Fb with decrease in oxidation state is not convincingly demonstrated. Knowledge of the impact of oxidation state on the nonbridging interatomic distances is even less well defined. To respond to these questions, and to the related one of the impact of oxidation state on the bridging angle M-Fb-M, a reliable set of accurate structures for the penta-, tetra-, and trifluorides of one of these metals was needed. 

Fluoride-ion capture from their anion relatives in anhydrous hydrogen fluoride solution by strong fluoride ion acceptors such as AsFb provides a general approach to the synthesis of polymeric binary fluorides and is particularly advantageous in the synthesis of highest-oxidation-state transition metal polymeric fluorides. 

Other fluorofullerenes are obtained by reaction with metal fluorides. These may be less reactive than elemental fluorine, but consequently enable a much better selectivity in product generation. Depending on the metal chosen (either transition or rare earth metals might be considered), the composition of the fluorine compound ranges from C )F2 to about C60F36. Not only binary, but also ternary metal fluorides can be employed (Table 2.10). 

We have already seen in Chapter 4 that the reduced one-bond coupling constants (XF) for the binary fluorides have a periodic behavior with atomic number of the X element, which follows along the periodicity of the density of the wave function at the nucleus. The general trends of (AF) in AFj L , in tetrahedral AF4 L , and octahedral AFg L compounds have been summarized, A being main group element as well as transition metal. 

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