= Solution
\b[Primary amines can favor geminal disubstitution through a proton-abstraction route.] The first substitution leaves an exocyclic $\mathrm{PCl(NHR)}$ group. Its remaining N–H <proton> provides a pathway unavailable to a fully substituted amino group: base removes H, chloride elimination gives a reactive low-coordinate phosphorus intermediate with increased exocyclic P–N bonding, and a second amine attacks that same phosphorus. <proton> transfer then gives $\mathrm{P(NHR)_2}$. Thus the directing effect is not simply the inductive effect of an amino group.
This needs a qualification: “primary amine” alone does not specify a universal product distribution. Substituent, solvent and base concentration affect whether the proton-abstraction route outcompetes direct <nucleophilic substitution>. Kinetic and trapping experiments with aromatic primary amines support the base-catalyzed route to geminal products, while also finding direct substitution pathways under other conditions.<Studies of Phosphazenes. Part 31 by Sivapackia Ganapathiappan and Setharampattu S. Krishnamurthy>[1987, p. 585 abstract, solvent/base dependence and methanol trapping]{ref}
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