C-H...X (X = F, Cl, Br, I) Versus π-Stacking in the Crystal Packing of Compounds Containing the {M(tpy)X₃} Motif

Analysis of the data in the Cambridge Structural Database (CSD) for compounds containing an {M(tpy)X 3 } motif (tpy = 2,2':6',2"-terpyridine, M = any metal, X = F, Cl, Br, I) reveals 17 isostructural mononuclear [M(tpy)X 3 ] compounds crystallizing without lattice solvent; both face-to-face π-stacking of pyridine rings and C-H3/H3' . . . X hydrogen bonding appear to be equally important. Regardless of coordination number (CN = 6, 7 or 8) and nuclearity (mono- or dinuclear), a recurring packing feature in other compounds containing an {M(tpy)X 3 } unit is the presence of bifurcated Cl . . . H3/H3' interactions, complemented in some cases by Cl . . . H5'/H3" interactions, consistent with the acidic H3, H3', H5', and H3" atoms of a coordinated tpy ligand. Octahedral [M(tpy)F 3 ] complexes crystallize as hydrates with strong F . . . H-OH hydrogen bonding dominating the crystal packing