Halides

Molecular BX₃ compounds are created with all halogens. They contain the trigonal planar structure (D_3h) predicted through VSEPR (see Topics C2, C3), even though there show to be a specific degree of π bonding (strongest in BF₃) involving halogen lone-pairs and the empty boron 2p orbital (see 2 for one of the possible forms of resonance). The halides are strong Lewis acids, BF₃ and BCl₃ being employed as catalysts (example in organic Friedel-Crafts acylations). Interaction with a donor gives a tetrahedral geometry approximately boron as along with the analogous BH₃ complex 1. The π bonding in the parent molecule is lost and for this cause BF₃, where such type of bonding is strongest, is more resistant to adopting the tetrahedral geometry than are the heavier halides. So the acceptor strengths follow the order

BF₃ < BCl₃< BBr₃< Bl₃

that is the opposite of that found with halides of most other elements

Strongest interaction take place with hard donors like F^- (forming the stable tetrafluoroborate ion [BF₄]^-) and with oxygen donors like water. Apart from with BF (in which the B-F bonds are very strong) complex formation frequently leads to solvolysis, forming B(OH)₃ in water. BF₃ itself creates a 1:2 aduct with water, that in the solid state can be formulated like [BF₃(H₂O)].H₂O, one water molecule being coordinated to boron through an oxygen lone pair and the other held independently through hydrogen bonding. On melting at 6°C an ionic liquid consisting of [H₃O]⁺ and [BF₃(OH)]^- is obtained. Pyrolysis of BX₃ compounds leads to halides with B-B bonds, for instance, B₂X₄ (3 with X=F or Cl) and polyhedral BnCln molecules (n=4, 8, 9).