We report the synthesis, crystal structures, thermal, IR, UV-vis, and magnetic properties of a series of divalenttransition metal formates, [NH
4][M(HCOO)
3], where M = divalent Mn, Co, or Ni. They crystallize in the hexagonalchiral space group
P6
322. The structure consists of octahedral metal centers connected by the
anti-anti formateligands, and the ammonium cations sit in the channels. The chiral structure is a framework with the rarely observed4
9·6
6 topology, and the chirality is derived from the handedness imposed by the formate ligands around the metalsand the presence of units with only one handedness. The thermal properties are characterized by a decompositionat ca. 200
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C. The three compounds exhibit an antiferromagnetic ground state at 8.4, 9.8, and 29.5 K for Mn, Co,and Ni, respectively. The last two display a weak spontaneous magnetization due to a small canting of the momentsbelow the critical temperature, and the Co compound shows a further transition at lower temperatures. The isothermalmagnetizations at 2 K show spin-flop fields of 600 Oe (Mn), 14 kOe (Co), and above 50 kOe (Ni) and a smallhysteresis with a remnant magnetization of 25 cm
3 G mol
-1 (Co) and 50 cm
3 G mol
-1 (Ni) and coercive field of400 Oe (Co) and 830 Oe (Ni).