Chemistry - Are the bonds within bismuth pentafluoride (BiF5) covalent or ionic?
Solution 1:
For all nomenclature purposes, one should refer to the official IUPAC recommendations, which in this case are laid down in Nomenclature of Inorganic Chemistry — IUPAC Recommendations 2005, the ‘Red Book’. The relevant section for this question is section IR-5: Compositional Nomenclature, and Overview of Names of Ions and Radicals
The introduction of this section (IR-5.1) states:
The simplest type of compositional name is a stoichiometric name, which is just a reflection of the empirical formula (Section IR-4.2.1) or the molecular formula (Section IR-4.2.2) of the compound. In stoichiometric names, proportions of constituent elements may be indicated in several ways, using multiplicative prefixes, oxidation numbers or charge numbers.
The following details on Stoichiometric Names of Elements and Binary Compounds (section IR-5.2) detail how to derive a stoichiometric name:
The stoichiometric name of the compound is then formed by combining the name of the electropositive constituent, cited first, with that of the electronegative constituent, both suitably qualified by any necessary multiplicative prefixes (‘mono’, ‘di’, ‘tri’, ‘tetra’, ‘penta’, etc., given in Table IV). The multiplicative prefixes precede the names they multiply, and are joined directly to them without spaces or hyphens. The final vowels of multiplicative prefixes should not be elided (although ‘monoxide’, rather than ‘monooxide’, is an allowed exception because of general usage). The two parts of the name are separated by a space in English.
Thus, the stoichiometric name of $\ce{BiF5}$ is bismuth pentafluoride.[1]
In the later section Generalized Stoichiometric Names, guidance is given on how to name compounds based on one constituent’s charge or oxidation number (IR-5.4.2.2).
It is possible to provide information on the proportions of the constituents in names by using one of two other devices: the charge number, which designates ionic charge, and the oxidation number, which designates oxidation state. […]
[…]
The oxidation number (see Sections IR-4.6.1 and IR-9.1.2.8) of an element is indicated by a Roman numeral placed in parentheses immediately following the name (modified by the ending ‘ate’ if necessary) of the element to which it refers. The oxidation number may be positive, negative or zero (represented by the numeral 0). An oxidation number is always non-negative unless the minus sign is explicitly used (the positive sign is never used). Non-integral oxidation numbers are not used for nomenclature purposes.
Thus, according to this nomenclature, $\ce{BiF5}$ (determination of bismuth’s oxidation state is unambiguous) is bismuth(V) fluoride.
These two different styles of naming binary compounds are equivalent. In no way do they include any statement concerning the bonding situation. In fact, these names explicitly do not contain any information concerning structure or connectivity: dinitrogen tetraoxide is the correct name for both $\ce{O2N-O-N-O}$ and $\ce{O2N-NO2}$ — and the generalized name nitrogen(IV) oxide is also correct for $\ce{NO2}$.
Notably, usage of one or the other nomenclature cannot be used to distinguish between covalent or ionic bonding. As a case in point, $\ce{CCl4}$ — a rather clearly covalently bonded molecule — can be referred to as:
- carbon tetrachloride
- carbon(IV) chloride
And vice-versa, $\ce{BaCl2}$ — a very ionic compound — can be referred to as:
- barium dichloride
- barium(II) chloride
- barium chloride[1]
Therefore, neither bismuth pentafluoride nor bismuth(V) fluoride make any reference to the type of bonds present in the compound; it is up to the chemist to use their previously acquired knowledge to realise whether bonds are predominantly covalent or predominantly ionic or something else.
Note:
Multiplicative prefixes need not be used in binary names if there is no ambiguity about the stoichiometry of the compound […]. The prefix ‘mono’ is, strictly speaking, superfluous and is only needed for emphasizing stoichiometry when discussing compositionally related substances […].
(Section IR-5.2)
Therefore, bismuth pentafluoride is permitted alongside monobismuth pentafluoride; and barium chloride is permitted, since only barium dichloride is reasonable.
Solution 2:
Nothing is completely ionic or completely covalent; it's just a label.
Convention is that electronegativity difference defines the nature of the bond, with 1.7 being the determining number.
However, if you look up a table, the electronegativity difference between bismuth and fluorine is 1.96, which although is conventially ionic, is really between the two.
Hence, it gets named as bismuth pentafluoride as well as bismuth (V) fluoride (this name is also on the same Wikipedia page).