A series of novel inclusion compounds with compositions of RE(HCO2)(3)(HNO2)( H2CO2) (RE = Y, Tb, Dy, Ho, Er, Yb, Tm) were synthesized and structures were characterized by X-ray methods. The orthorhombic structure was shown to be a new type with a non-centrosymmetric space group (20) C222(1) by single crystal structure determinations. It is characterized by networks of rare earth centered square anti-prisms formed by eight oxygen atoms through bridging carbon and nitrogen atoms. The guest formic acid molecules H2CO2 are distributed inside the open tunnels along the crystallographic a axis. The magnetic susceptibility measurements show the heavy rare earth compounds follow the Curie-Weiss law and the calculated numbers of Bohr magnetons are consistent with the RE3+ ions. The yttrium compound shows very weak temperature independent paramagnetism
