The packing of molecular crystals, in which the constituent molecular units have no directional forces, is primarily controlled by weak London dispersion (LD) forces. These forces assist in stabilizing the system by bringing the molecular units into the proximity of each other. In this paper, the same effect is shown to be externally induced by pressure. The minimal pressure required to correctly describe the crystal structure without LD interactions (P LD) provides a quantifiable measure for the weak intermolecular interactions. LD forces are shown to be essential for an accurate description of the pressure-induced phase transitions across examples of linear, trigonal-planar, square-planar, tetrahedral, trigonal bipyramidal, and octahedral molecules.

Iron carbonyls are known to form 18-electron complexes like Fe­(CO)5, Fe2(CO)9, and Fe3(CO)12 having terminal or bridged Fe–CO bonding. Based on genetic algorithm-assisted density functional theory (DFT) calculations, it is predicted that at pressures above 2 GPa, iron tetracarbonyl, Fe­(CO)4, attains a square-planar geometry with a 16-electron count. Compression overcomes the [Ar]­4s23d6 (S = 2) → [Ar]­4s03d8 (S = 0) excitation energy to stabilize a closed-shell Fe­(CO)4 with a d8-configuration. Strong σ­(4CO) → Fe (d x 2 –y 2) bonding along with Fe­(d xz , d yz ) and Fe­(d xy ) → π (CO)4* back-bonding assists the formation of square-planar Fe­(CO)4 under pressure. Compression progressively flattens and destabilizes the ambient pressure C2v structure of Fe­(CO)4, and beyond 2 GPa, it undergoes a sharp C2v → D4h transition with ΔV unit‑cell = 2.1% and trans-θ­(OC–Fe–CO) = 180°. Realizing a square-planar geometry in a four-coordinated Fe-carbonyl complex shows the rich prospects of the new chemistry under pressure.

A conformationally flexible ligand [(Flu)-(CH2)2-(NHC)-CH2-(NHC)-(CH2)2-(Flu)]2- (L) is devised that offers four sequential organic donors modules and enwraps a Ca2+ pseudo-tetrahedrally in a twisted ‘S’-shape. The [(L)Ca] synthesis is intriguingly stepwise...