To understanf the ncomplete octet phenomena of boron, let us look at the lewis structure for BH₃ structure:

The problem with this structure is that boron has an incomplete octet; it only has six electrons around it. Hydrogen atoms can naturally only have only 2 electrons in their outermost shell (their version of an octet), and as such there are no spare electrons to form a double bond with boron. One might surmise that the failure of this structure to form complete octets must mean that this bond should be ionic instead of covalent. However, boron has an electronegativity that is very similar to hydrogen, meaning there is likely very little ionic character in the hydrogen to boron bonds, and as such this Lewis structure, though it does not fulfill the octet rule, is likely the best structure possible for depicting BH₃ with Lewis theory. One of the things that may account for BH₃ incomplete octet is that it is commonly a transitory species, formed temporarily in reactions that involve multiple steps.