How is chirality center defined for an atom with 5 substituents?
Chirality depends not so much on coordination number, but on the symmetry of the substituents.  Meaning you have to pay attention to coordination geometry.  In the case of four-coordination you can have four different substituents and yet no chirality... if you have square planar instead of tetrahedron coordination.
For five-coordination you lack a mirror plane and thus have a chiral center in the following cases:

Square pyramidal geometry: if all four substituents on the base are different, or if just one pair of these basal substituents are identical and the identical pair is on adjacent vertices.  The axial substituent does not have any impact because all possible mirror planes pass through it.

Triangular bipyramid geometry: if all three equatorial substituents are different and the two axial substituents are different from each other.  You may still have chirality if one axial substituent happens to be the same as one equatorial one.