The title compound crystallizes in the tetragonal space group I41/a, and its molecular structure in the solid state shows an adamantoid core decorated with four exo N-SiMe3 substituents. The two most striking features of the structure are i) the very short P=N bond length of 1.500(3) (Å), which is shorter than 98% of all P=N double bond length values reported (c.a. 15, 000 in the Cambridge Structural Database) and ii) the obtuse P=N-Si angle [173.9(3) o ], which is larger than the values found in >95% of all silylphosphinimines (R3P=NSiR3, see Figure , SI). In contrast, the density functional theory calculated geometry of an isolated molecule of 2 shows a P=N bond length of 1.546 Å and a P=N-Si angles of 132 o , in line with expectations for an imine. We conclude that the experimentally observed bond angle distortion arises from intermolecular forces in the lattice. A view of the sub van der Waals interactions between molecules reveals no interactions involving any of the skeletal P or N atoms, but rather numerous contacts between peripheral Me groups on adjacent units (Figure , SI). Given that SiR3 groups and methyl groups adjacent to heteroatoms are known to be very polarizable, we interpret the lattice energy in 2 as being primarily a result of dispersion forces. Such dispersion-held lattices are ubiquitous in hydrocarbon chemistry, but their dominance is unexpected in heteroatom-dense species like 2, featuring a high number of lone pairs (10), double bonds (4), and polar σ bonds (12 P-N, 4 Si-N bonds). In this context, the title compound is electronically quite distinct from the well-known tetrahedral scaffolds B, which lack polarizable skeletal lone pairs, but it exhibits similar intermolecular forces and physical properties due to symmetry. For example, compound 2 is soluble in all tested hydrocarbon, ethereal, halocarbon, nitrile, and aromatic solvents and sublimes at ca. 150 o C at atmospheric pressure. irradiation or heating at 120 o C in the presence of 10-fold excess of azobis(isobutyro)nitrile (AIBN). This extent of redox inertia is surprising given the reported UV and oxidative sensitivity of phosphinimines. 39- Scheme 2. Robustness of 2 under various conditions. ODCB = ortho-dichlorobenzene, AIBN = azobis(isobutyronitrile).