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The count of atoms/ions/moleculesthat a central atom/ion can hold in a particular complex is termed as the coordination number.It is evaluated by counting the neighboring atoms around a central atom.

Boron is limited to a coordination number of 4 because of its size. Its atomic number is 5 and its electron configuration is \({\rm{1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{p}}^{\rm{1}}}\). With a small number of protons and electrons, it has a very small radius.

From the electron configuration, it is clear that borononly has the s and p orbitals and no d orbital. A maximum of four pairs of electrons can be present in these orbitals.

For these reasons, there can't be more than four atoms surrounding aboron atom.