In octahedral structure MX6, the six hybrid orbitals (sp3d2) are directed towards the corners of a regular octahedral with an angle of 90∘. According to following structure of MX6, the number of X−M−X bonds at 180∘ must be three. 
Related Theory In octahedral, two orbitals contain lone pairs of electrons on opposite sides of the central atom. The remaining four atoms connected to the central atom give the molecule a square planar shape.