Cesium 5-cyano-1,2,3,4-tetrazolate, Cs[NCCNNNN] (Cs1), was prepared in 100% yield by a 3 + 2 cycloaddition reaction of CsN3 and (CN)(2) in SO2. Cs1 forms monoclinic crystals (a = 8.297(2) Angstrom, b = 11.040(3) Angstrom, 6.983(2) Angstrom, beta = 120.31(2)degrees space group C2/c, Z = 4, R1 = 0.048, wR2 = 0.120 for 1217 independent reflections). Cs1 is best described as a three-dimensional array of cations and anions connected by weak Cs+-Ndelta- contacts. The cations and anions each form a diamond-type lattice (tetrahedral arrangement of ions) with the counterions lying in hexagonal channels running parallel to the c-axis. The anions in the channels form stacks with the CN groups pointing in opposite directions in adjacent layers. The calculated (RB3PW91/6-311+G*) geometry of 1 is in agreement with the X-ray crystal structure, and the calculated vibrational spectrum is in good agreement with the observed FT-IR and FT-Raman spectra. The lattice enthalpies and heats of formation of M1 (M = Cs, K) as well as the enthalpy of formation from MN3(s) and (CN)(2)(g) were estimated. The C-13 and N-14 NMR spectra of Cs1 are also reported. The ionization potential (450.7 kJ mel) and electron affinity (427.4 kJ/mol) of 1 were calculated. Attempts to oxidize 1 with AsF5 led to the formation of Cs1 . xAsF(5) (x approximate to 2). The 7 pi dianion I I [NCCNNNN](. 2-) is calculated to be a stable entity in the gas phase, but Cs(2)1 is estimated to be unstable with respect to dissociation to 2 CsCN and 3/2 N-2 (Delta H-diss = -132.4 kJ/mol). The preparation of the potassium salt of 1 and the corresponding thermodynamic quantities are reported.