Abstract
<jats:p>Rotation about the phosphorus–nitrogen (P–N) bond in aminophosphines influences their conformational preferences, donor environments, and reactivity. Here, density functional theory calculations, energy decomposition analysis, and natural bond orbital methods are used to examine how substitution at phosphorus and nitrogen controls P–N rotational barriers across a series of aminophosphines. The results show that the barriers cannot be described by substituent size or any single bonding interaction alone. Instead, rotation arises from coupled changes of nitrogen pyramidalization, lone-pair localization, P–N bonding, and anomeric-type donation from the nitrogen lone pair into phosphorus-centered antibonding orbitals. In the more congested derivatives, reorganization of the nonbonded environment introduces additional electrostatic contributions that depend on the three-dimensional arrangement of the substituents rather than on steric bulk alone. Together, these effects produce strongly substituentdependent rotational barriers and show why structurally similar aminophosphines can exhibit markedly different conformational behavior. These findings clarify the physical origins of P–N rotational barriers and provide a framework for tuning the flexibility and donor properties of aminophosphines through substitution.</jats:p>