Secondary bonding interactions play a major role in governing the overall structures adopted. The low energy contributions from these weak interactions make structure prediction very difficult, hence there is a need for experimental techniques that contribute to understanding the interplay between different types of secondary bonding. Variable pressure single crystal X-ray diffraction studies on the homologous series, [GeX 2 {o-C 6 H 4 (PMe 2 ) 2 }], X = Cl 1, Br 2, I 3, show that probing the

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... fferent interfaces between layers of structural building blocks, rather than conventional molecular units, provides very valuable insights. 1 and 3 undergo a smooth compression as the pressure is increased, whereas a phase transition occurs for 2 at a pressure between 29 and 41 kbar. This is associated with an abrupt change in the β angle (from 111.33(2)°to 92.24(8)°). The structural consequences are most evident in the aromatic⋯aromatic layer interface. Below the phase transition there is an edge-to-face C-H⋯π arrangement (like 1), with the angle between the planes of adjacent rings of~75°, whereas above the transition this interface has transformed to an offset-parallel face-to-face π-π stacking interaction (like 3). The GeX 2 ⋯X 2 Ge interface undergoes a concomitant, but smoother compression with increasing pressure. 2 also has the highest void volume at ambient pressure (11.9%), and as expected the phase transition results in a structure with much more efficient packing. This, the first such study involving p-block coordination complexes, reveals the subtlety and complexity of the interplay between the different forms of weak, secondary (supramolecular) interactions present. The results indicate that this type of experimental study can provide valuable additional information to help guide crystal structure prediction by computational methods, an important and very challenging target. CrystEngComm, 2014, 16, 8169-8176 | 8169 This journal is and Innovation Campus, Didcot OX11 0QX, UK † Electronic supplementary information (ESI) available. CCDC 977894-977899 for 1; CCDC 977912-977920 for 2. For ESI and crystallographic data in CIF or other electronic format see