text: In 2007 Houser et al. developed the analogous  parameter to distinguish whether the geometry of the coordination center is square planar or tetrahedral. The formula is:
  where:  and  are the two greatest valence angles of coordination center;  is a tetrahedral angle.
  When  is close to 0 the geometry is similar to square planar, while if  is close to 1 then the geometry is similar to tetrahedral. However, in contrast to the  parameter, this does not distinguish  and  angles, so structures of significantly different geometries can have similar  values. To overcome this issue, in 2015 Okuniewski et al. developed parameter  that adopts values similar to  but better differentiates the examined structures:
  where:  are the two greatest valence angles of coordination center;  is a tetrahedral angle.
  Extreme values of  and  denote exactly the same geometries, however  is always less or equal to  so the deviation from ideal tetrahedral geometry is more visible. If for tetrahedral complex the value of  parameter is low, then one should check if there are some additional interactions within coordination sphere. For example, in complexes of mercury(II), the Hg···π interactions were found this way.