Boron forms several different complex anions. Which one of the following boron containing anions cannot normally be formed or isolated under standard chemical conditions?

Introduction / Context:
Boron chemistry includes a variety of oxoanions and complex anions with ligands such as hydroxide, fluoride and hydride. Some of these species are common and well characterised, while others are not chemically reasonable under normal conditions. Exam questions like this one test whether you can distinguish realistic boron anions from formulas that are unlikely or incorrect, based on your knowledge of common boron compounds and coordination tendencies.

Given Data / Assumptions:

• The anions listed are BO2-, BF63-, BH4- and B(OH)4-.
• We assume typical aqueous or standard laboratory conditions.
• We rely on standard inorganic chemistry knowledge of borate, borohydride and borate related anions.
• The task is to identify which formula does not correspond to a stable or commonly known boron anion.

Concept / Approach:
Boron commonly forms oxoanions such as metaborate (BO2-) and tetrahydroxoborate (B(OH)4-), as well as hydride complexes like borohydride (BH4-). These anions are well known in inorganic and coordination chemistry. Fluoride complexes of boron do exist, but a very common and stable example is BF4-, not BF63-. The formula BF63- would involve six fluoride ligands around boron with a 3 negative charge, which is not a standard or stable coordination environment for boron. By recognising which anions are well established, you can spot the odd one out that is not normally formed.

Step-by-Step Solution:
Step 1: Consider BO2-. In borate chemistry, BO2- is a reasonable representation of metaborate, which can be found in salts such as sodium metaborate.Step 2: Consider B(OH)4-. This is the tetrahydroxoborate ion, commonly formed when boric acid reacts with hydroxide in aqueous solution.Step 3: Look at BH4-, known as the borohydride ion. It is widely encountered in sodium borohydride and other hydride reducing agents used in organic and inorganic reactions.Step 4: Examine BF63-. The most familiar boron fluoride complex in introductory inorganic chemistry is BF4-, the tetrafluoroborate ion, not BF63-.Step 5: Six coordination around boron with such a high negative charge is not consistent with the usual valence and bonding patterns of boron in simple compounds.Step 6: Therefore, BF63- is the anion among the options that boron does not normally form.

Verification / Alternative check:
You can cross check by recalling typical boron compounds that appear in textbooks or laboratory reagents. Borax, boric acid derivatives and borate salts involve BO3- or BO2- related units and occasionally B(OH)4-. Reducing agents such as sodium borohydride contain BH4-. Fluoroborate salts like sodium tetrafluoroborate contain BF4-. There is no standard example of a stable BF63- boron complex introduced at this level. This pattern confirms that BF63- is the unusual and chemically unreasonable formula in the list.

Why Other Options Are Wrong:
Option a, BO2-, corresponds to the metaborate ion, which is a recognised boron oxoanion. Option c, BH4-, is the well known borohydride ion used widely in reduction reactions. Option d, B(OH)4-, is a common anion formed when boric acid interacts with strong bases in water. Since all of these appear in standard boron chemistry, they cannot be the answer to a question asking which anion boron cannot form.

Common Pitfalls:
A common mistake is to focus only on the charge and assume that any unusual charge makes an ion impossible, without considering known examples like BH4-. Another pitfall is to confuse BF4- with BF63- and assume that any fluoroborate formula is acceptable. To avoid such errors, pay attention to the coordination number and recall at least a few concrete examples of boron anions you have encountered. When an unfamiliar formula appears that does not match known patterns, that is a strong clue that it is the incorrect one.

Final Answer:
The boron containing anion that is not normally formed is BF63-.