Final answer:
BH3 is electron deficient due to boron having only six valence electrons, promoting the formation of coordinate covalent bonds to achieve a stable octet through its trigonal planar sp² hybridized structure.
Step-by-step explanation:
The addition of BH3, or borane, occurs in a unique manner due to the electron deficiency of the boron atom. Boron has only six valence electrons, making it electron deficient and susceptible to forming adducts with molecules that can donate a pair of electrons. The BH3 molecule is trigonal planar with sp² hybridization due to the redistribution of boron's three valence electrons into three sp² hybrid orbitals. This hybridization allows for the formation of three sigma bonds with the hydrogen atoms. Despite an incomplete octet, these covalent bonds provide the best possible structure for BH3 under Lewis theory. Additionally, its reactive nature means that it readily forms coordinate covalent bonds with molecules like (CH3)2S and others that have available lone pairs. In reaction mechanisms, an electron-deficient molecule like BH3 seeks to complete its octet by bonding with molecules that have available electron pairs. This is also seen with BF3, where the boron atom often forms additional bonds as in BF4 to achieve a full octet, which demonstrates that configurations fulfilling the octet rule are usually more stable.