Revisiting Metal Dihalide Complexes with Phosphine Ligands: Isostructurality, Steric Effects, Anagostic Interactions, Crystal Voids, and Energetic Calculations

Abstract The crystal structures of seven metal dihalide complexes with triphenylphosphine (PPh₃) and two with tribenzylphosphine (PBz₃) have been reanalyzed. The studied complexes include [FeCl₂(PPh₃)₂], [CoCl₂(PPh₃)₂], [CoBr₂(PPh₃)₂], [NiCl₂(PPh₃)₂], [NiBr₂(PPh₃)₂], [NiI₂(PPh₃)₂], [ZnCl₂(PPh₃)₂], [NiCl₂(PBz₃)₂], and [NiBr₂(PBz₃)₂]. A detailed geometric analysis revealed high isostructurality among several pairs. Ligand steric effects were assessed using cone angles and percentage surface coverage. Crystal packing is stabilized by weak C─H⋯π and C─H⋯X (X = Cl, Br, and I) interactions. Notably, [NiCl₂(PBz₃)₂] and [NiBr₂(PBz₃)₂] feature rare intramolecular C─H⋯Ni anagostic interactions. Reduced density gradient (RDG) and noncovalent interaction (NCI) plots confirmed weakly attractive C─H⋯π contacts in all complexes. Void‐volume analysis assessed the mechanical strength of the crystals, while pixel energy and energy frameworks clarified structural stability and dominant interactions. Density Functional Theory (DFT) calculations, along with QTAIM/NCIplot and NBO analyses, as well as electron density (ED) versus electrostatic potential (ESP) plots, were performed to better understand the C─H···Ni interactions and distinguish between agostic and anagostic interactions.