Both the radius ratio condition and the relative covalent charecter should be taken into account for deciding the structure of border line compound-comment.

Radius ratio for the length of the circuference both the atoms touching that is to say that the perimeter or the circumference it tends to touch and being in contact with each other.
Covalent character is vital for the strength of the atom and for the penetration or the pushing effect or the combining or the attraction force being held between the two atoms.