Question: why bif3 is predominantly covalent?
Answer: Trihalides except BiF3 are predominantly covalent on account of Fajan's rule. Small size of cation, large size of anion, and high charge on both species results in greater covalent character. BiF3 is ionic because the size cation (Bi) is quite larger amd size of anion (F) is smaller. ... Hence BiF3 is ionic.