Group 13 elements have the ability to form anion adducts with Lewis bases because they can form trigonal planar covalent compounds that are Lewis acids (BF3 + F- BF4-) 1. The tetrafluroborate anion can be used to form ionic metal complexes because it is a non-coordinate anion. A halide salt of a transition-metal cation is prepared to allow for an anion exchange from a salt of tetrafluoroborate to form a tetrafluoroborate salt of the transition-metal cation.
Boric acid reacts with ammonium fluoride, with concentrated sulfuric acid as a catalyst, to form ammonium tetrafluoroborate (H3BO3 (aq) + 2 NH4HF2 (aq) NH3 (aq) + 3 NH4BF4 (aq) + 3 H2O (l)). Boron in boron triflouride is missing two valence electrons for an octet, making it easy to hydrolyze. The tetrafluoroborate anion is possible and also very stable; that is why tetrafluoroborate is considered a weakly coordination anion. The tetrafluorobrate anion bind well to electrophilic metals.
Dilute (2 M) sulfuric acid (7.5 mL) and water (15 mL) was added in a 100 mL beaker. This solution was added to boric acid (3.097 g), then was heated and stirred on a hot plate until the solution was clear. Small portions of ammonium fluoride (7.450 g) were added to the solution. The beaker was covered with a watch glass and stayed on the steam bath for 30 minutes to complete the reaction. The solution as cooled in an ice-water bath (̴̴ 15 min.) then concentrated (6 M) sulfuric acid (7.5 mL) was added to the solution. Once again the solution was cooled, this time for 20 minutes. Vacuum filtration was used to obtain the solid. The vacuum was turned off then acetone (7.5 mL) was used to wash the solid. The crystals were vacuumed once again and washed further with acetone (5 mL). It was allowed to dry at room temperature and the weight was recorded. This was product A. Nickel (II) chloride hexahydrate (3.065 g) was dissolved in water (5 mL), and then placed in the hot plate to be warmed. Dilute (6 M) ammonia was added until the solution turned blue (̴ 20 mL). Water (8 mL) was added and the solution was filtrated to collect the filtrate (which contained hexaamminenickel (II) ion). Product A (2.5 g) and 2 M ammonia (7.5 mL) was heated for 3-4 minutes. This solution as vacuum filtrated and its filtrate was collected in the beaker containing hexaamminenickel (II) ion. The solution was cooled in an ice-water bath (10-15 minutes).
Results: Table 1. The recorded weight and yield of product A and product B.
Weight of watch glass and filter paper
Weight of watch glass, filter paper, and product A
Yield of Product A
Weight of watch glass, filter paper, and product B
Yield of Product B
41.153 - 35.190 = 5.963
Discussion: Tetrafluoroborate (BF4 -) is a tetrahedral, non-polar compound. It consists of one boron (sp3 hybridized) and 3 fluorine atoms. Tetrafluoroborate is a nucleophile and a Lewis base, but it is weaker than halides regarding basicity. Reactions involving salts of tetrafluoroborate, the anion is considered inert due to its symmetry (the negative charged is evenly distributed between the 4 atoms) and the presence of high electronegative fluorine atoms (which makes it less likely to donate electrons). A yield of product A was produced, but that correct compound was not made. The crystallization did not occur because dilute sulfuric acid was used in the solution. To compensate for the wrong addition of dilute sulfuric acid, concentrated sulfuric acid was added and crystallization was observed. Product B was not formed because the beaker containing hexaamminenickel (II) ion was heavily diluted