Final answer:
The maximum coordination number of four in boron compounds is a result of electron deficiency.
Step-by-step explanation:
The maximum coordination number of four in boron compounds is a result of electron deficiency. Boron has one less valence electron than the number of valence orbitals it has. This electron deficiency allows boron to form stable compounds with less than eight electrons around the atom, often resulting in a coordination number of four.