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 merge with the singly occupied orbitals of radical ligands (e.g., oxygen), or the introduction of a strong field ligand can lead to electron pairing, creating an available orbital for donation.