These singly occupied orbitals can combine with the singly occupied orbitals of radical ligands (e.g., oxygen), or addition of a strong field ligand can cause  electron-pairing, thus creating a vacant orbital that it can donate into.
Singly occupied orbitals can combine with the singly occupied orbitals of radical ligands like oxygen, or the addition of a strong field ligand can cause electron pairing, resulting in an empty orbital that can be donated by the ligand.