How does each of the following modifications affect the rate of a square-planar complex substitution reaction? 

(a) Changing a trans ligand from H⁻ to Cl⁻; 

(b) Changing the leaving group from Cl⁻ to I⁻; 

(c) Adding a bulky substituent to a cis ligand;

(d) Increasing the positive charge on the complex.