Shown on the following page is the log of the rate constant for substitution of by phosp...

Shown on the following page is the log of the rate constant for substitution of by phosphorus and nitrogen ligands, plotted against the half-neutralization potential (?HNP) of the ligands. ?HNP is a measure of the basicity of the compounds. Explain the linearity of such a plot and why there are two different lines. Incoming nucleophilic ligands:

(12) 4-picoline, (13) pyridine, (14) a-chloropyridine. (Reproduced with pernaission from E. M.Thorsteinson and E Basolo, J. Am. Chem. Sac., 1966, 88, 3929. © 1966 American Chemical Society)