The enhancement of donor strength of ortho-functionalized triarylphosphanes is shown to occur via different mechanisms for O- and N-donor substituents, with internal solvation of the phosphorus center observed for N donors. Nevertheless, the steric congestion about the P center is shown to significantly oppose the increase in donor ability, leading to donation weaker than that expected. A series of mono- and bis-aryl-substituted Ar3PI2 adducts (Ph3-n(o-OMe-C6H4)nPI2, Ph3-n(o-NMe2-C6H4)nPI2, Ph3-n(o-CH2NMe2-C6H4)nPI2 (n = 1, 2)) have been synthesized via the 1:1 reaction of donor-functionalized phosphanes with diiodine. These soft Lewis acid/base adducts exhibit apparent internal solvation of the donor phosphorus by the pendant donor moieties, giving rise to five- or six-coordinate phosphorus atoms acting as both Lewis base and Lewis acid; the first neutral six-coordinate simultaneous P(III) Lewis acid and Lewis base adduct is reported. Single-crystal X-ray diffraction studies reveal unexpectedly weak donor strength for one of the phosphanes, indicating significant steric hindrance as a consequence of internal solvation. Crystallographic interrogation of the corresponding iodophosphonium salts [Ar3PI]X (X = I3, BArF) shows that the cationic complexes experience a still greater influence of the steric bulk of the donor moieties than their neutral precursors. The steric and electronic contributions to bonding have been analyzed through computational studies, determining the factors governing the basicity of these donor-functionalized phosphanes, and show that enhancement of P-centered donor strength occurs by conjugation of lone pairs through the arene rings for oxygen substituents and via internal solvation for the nitrogen donors.