The diphosphatetrylenes {(Dipp)(R')P}(2)E [R' = Mes, E = Ge (1Ge), Sn (1Sn); R' = CH(SiMe3)(2), E = Sn (2Sn)] have been isolated and characterized by multinuclear and variable temperature NMR spectroscopy and X-ray crystallography [Dipp = 2,6-iPr(2)C(6)H(3), Mes = 2,4,6-Me3C6H2]. All three compounds crystallize as discrete monomers with two pyramidal phosphorus centers. However, variable-temperature P-31{H-1} NMR spectroscopy indicates that both 1Ge and 2Sn are subject to dynamic exchange between this form and a form containing one planar and one pyramidal phosphorus center in solution. In contrast, 1Sn retains two pyramidal phosphorus centers in solution and exhibits no evidence for exchange with a form containing a planar phosphorus center. The related compound [{(Me3Si)(2)CH}(2)P](2)Sn (3Sn) was isolated in very low yield and was shown by X-ray crystallography to possess one planar and one pyramidal phosphorus center in the solid state. DFT calculations reveal that the conformations of 1Ge, 1Sn, and 2Sn observed in the solid state are significantly stabilized by the delocalization of electron density from the aromatic rings into the vacant p-orbital at the tetrel center. Thus, for diphosphatetrylenes possessing aromatic substituents at phosphorus, stabilization may be achieved by two competing mechanisms: (i) planarization of one phosphorus center and consequent delocalization of the phosphorus lone pair into the vacant tetrel p-orbital or (ii) pyramidalization of both phosphorus centers and delocalization of aromatic pi-electron density into the tetrel p-orbital. For 3Sn, which lacks aromatic groups, only the former stabilization mechanism is possible.