To distinguish whether the geometry of the coordination center is trigonal bipyramidal or square pyramidal, the τ5 (originally just τ) parameter was proposed by Addison et al.:[1] where: β > α are the two greatest valence angles of the coordination center.
In 2007 Houser et al. developed the analogous τ4 parameter to distinguish whether the geometry of the coordination center is square planar or tetrahedral.
However, in contrast to the τ5 parameter, this does not distinguish α and β angles, so structures of significantly different geometries can have similar τ4 values.
To overcome this issue, in 2015 Okuniewski et al. developed parameter τ4′ that adopts values similar to τ4 but better differentiates the examined structures:[3] where: β > α are the two greatest valence angles of coordination center; θ = cos−1(− 1⁄3) ≈ 109.5° is a tetrahedral angle.
If for tetrahedral complex the value of τ4′ parameter is low, then one should check if there are some additional interactions within coordination sphere.