Talanra, Vol. 30, No. 8, pp. 579-586, 1983 0039-9140/83 %3.00+0.00 Printed in Great Britain. All rights reserved Copyright 0 1983 Pergamon Press Ltd A COMPUTATIONAL APPROACH TO THE SPECTROPHOTOMETRIC DETERMINATION OF STABILITY CONSTANTS-II APPLICATION TO METALLOPORPHY RIN-AXIAL LIGAND INTERACTIONS IN NON-AQUEOUS SOLVENTS D. J. LEGGETT,* S. L. KELLY, L. R. SHIUE, Y. T. Wu, D. CHANG and K. M. RADISH Department of Chemistry, University of Houston, Houston, TX 77004, U.S.A. zyxwvutsrqponmlkjih (Received 29 September 1982. Accepted 14 February 1983) Summar-The ability of the computer program SQUAD to deduce a plausible equilibrium model, associated stability constants and spectra of individual species is described. The original version of SQUAD has been extensively modified and these changes are detailed. In particular a “user-friendly” method of data input has been implemented that simplifies familiarization with the program. Brevity of program code has been sacrificed in favour of the new data input and error-checking features of SQUAD, with beneficial results. The application of SQUAD to five non-aqueous metalloporphyrin-axial ligand interactions exemplifies the program’s ability to handle widely different types of equilibrium systems. The Benesi-Hildebrand method’ has been extensively used to evaluate the stability constants of metalloporphyrin-axial ligand complexes. For the general reaction M + L=ML (1) where M is any metal ion or metalloporphyrin and L is any ligand, the following equations may be written: C, = [M] + [ML] (2) C,. = [L] + [ML] (3) Blot = [MLIWIV-I (4) The subscripts for j refer to the number of metal ions, hydroxide or hydrogen ions, and ligand ions, respectively, associated with that stability constant. The studies are most often performed in a non- aqueous, non-donor solvent, in which case the middle subscript is then always zero. Making use of Beer’s law: A = +,,[M] + &JL] + cML[ML] it can be shown’ that (5) log (A - 4,) ~ = log CL + log j&o, (A, - A) (6) where A is the absorbance, at a preselected wave- *Author for correspondence. Present address: Dow Chem- ical USA, Texas Division, Freeport, TX 77541, U.S.A. length, of a solution having a known molarity of ligand, C, and known molarity of metal, C, , A, is the absorbance, measured at the same wavelength, of a solution where Cr = 0, and A, is the measured ab- sorbance of a solution for which CL>>&, so that the absorbance is constant with increasing CL. Thus a plot of the left-hand side of equation (6) against log CL should give a straight line which intersects the abscissa at -log CL, which will be equal to log fl,,,, . The Benesi-Hildebrand (B-H) method may be generalized to handle equilibrium systems in which the stoichiometry of the complex formed is unknown. In this situation equation (4) becomes BW = [MW[Ml[Ll and equation (6) becomes (7) log (A - 4) ~ = n log CL + log j&). W-A) (8) Thus the slope of the straight line in the log-log plot provides the stoichiometric coefficient for the com- plex. Inherent in this method is the assumption that only one complex is formed. This situation is not too frequently encountered and when there is more than one complex present the B-H plots of log [(A - A,)/(A, -A)] vs. log CL will show considerable curvature. It has been argued’ that the plots may still yield information concerning the stoichiometry and stability constants of the complexes, but except under 579