Electronic state of the organic salt (DI-DCNQI)(2)Ag, where DI-DCNQI is 2,5-diiodo-N,N'-dicyanoquinonediimine

(DI-DCNQI)(2)Ag where DI-DCNQI is 2,5-diiodo-N,N' -dicyanoquinonediimine has been investigated by electrical resistance, magnetic susceptibility, and H-1 NMR measurements, and found to be nonmetallic with a charge gap of similar to 490 K and with spin susceptibility showing localization of the pi-electron spins. Antiferromagnetic ordering is evidenced at a low temperature. This is a first case of the electron antiferromagnet among the DCNQI-metal family in which the pi electrons on the DCNQI columns have an antiferromagnetic state. The nature of a Mott insulator demonstrates the importance of electronic correlation in the rr electrons in the DCNQI systems.