Before revision: When is close to 0 the geometry is similar to square pyramidal, while if is close to 1 the geometry is similar to trigonal bipyramidal: 4-coordinate compounds File:Tau4 and tau4prime parameters for selected geometries.gif In 2007 Yang et al.
After revision: When is close to 0 the geometry is similar to square pyramidal, while if is close to 1 the geometry is similar to trigonal bipyramidal: 4-coordinate compounds File:Tau4 and tau4prime parameters for selected geometries.gif In 2007 Houser et al.