Relative free energy of complex formation of bismuth(lIO with Diphenyl diseleno­phosphate ion in gaseous phase has been calculated through extrathermodynamic method for the iust time in solving the problem arising out of a complicated equilibrium with the participation of organic ligand having a complex structure. A new bi-parameter equation correlating stability constants with Donor and Acceptor number of the solvents has been proposed. This approach can provide sound basis in the selection of optimum solvents for use in Analytical Chemistry and several inorganic prepara­tions. Some practical steps have been underlined.