Phase-pure NaCN<sub>3</sub>H<sub>4</sub> and KCN<sub>3</sub>H<sub>4</sub> were synthesized from molecular guanidine and elemental metal in liquid ammonia at room temperature and elevated pressure close to 10 atm. The crystal structures were determined at 100 K using single-crystal X-ray diffraction. Both compounds crystallize in the monoclinic system (<i>P</i>2<sub>1</sub>/<i>c</i>, No. 14) but are far from being isotypical. NaCN<sub>3</sub>H<sub>4</sub> (<i>a</i> = 7.9496(12) Å, <i>b</i> = 5.0328(8) Å, <i>c</i> = 9.3591(15) Å, β = 110.797(3)°, <i>Z</i> = 4) contains a tetrahedrally N-coordinated sodium cation while KCN<sub>3</sub>H<sub>4</sub> (<i>a</i> = 7.1200(9) Å, <i>b</i> = 6.9385(9) Å, <i>c</i> = 30.404(4) Å, β = 94.626(2)°, <i>Z</i> = 16) features a very large <i>c</i> axis and a rather complicated packing of irregularly N-coordinated potassium cations. In the crystal structures, the guanidinate anions resemble the motif known from RbCN<sub>3</sub>H<sub>4</sub>, that is, with one elongated C–<sup>(amino)</sup>N single bond and two shorter C–<sup>(imino)</sup>N bonds (bond order = 1.5) although the orientation of one N–H bond differs in the guanidinate anion of NaCN<sub>3</sub>H<sub>4</sub>. Both crystal structures and infrared spectroscopy evidence the presence of hydrogen-bridging bonds, and the vibrational properties were analyzed by <i>ab initio</i> phonon calculations
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