Influence of HF2− geometry on magnetic interactions elucidated from polymorphs of the metal-organic framework [Ni(HF2)(pyz)2]PF6 (pyz = pyrazine)Electronic supplementary information (ESI) available: Absorption spectrum, X-ray diffraction patterns, spin density plots of BS-states, and CIF for β-PF6. CCDC reference number 863843. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2dt30113j

A tetragonal polymorph of [Ni(HF 2 )(pyz) 2 ]PF 6 (designated β) is isomorphic to its SbF 6 -congener at 295 K and features linear Ni-FHF-Ni pillars. Enhancements in the spin exchange ( J FHF = 7.7 K), Néel temperature ( T N = 7 K), and critical field ( B c = 24 T) were found relative to monoclinic α-PF 6 . DFT reveals that the HF 2 − bridges are significantly better mediators of magnetic exchange than pyz ( J pyz ), where J FHF 3 J pyz , thus leading to quasi-1D behavior. Spin density resides on all atoms of the HF 2 − bridge whereas N-donor atoms of the pyz ring bear most of the density. Linear Ni-F-H-F-Ni pathways in tetragonal β-[Ni(HF 2 )(pyz) 2 ]PF 6 leads to enhancements in J FHF , T N and B c relative to its monoclinic α-phase.