New organic conductors which contain a magnetic cation of Co2+ (d(7), S=3/2), (BEDT-TTF)(2)MCo(SCN)(4) [M=K, Rb, Cs], and related materials were prepared. The crystal structure analyses clarify that alpha(BEDT-TTF)(2)MM'(SCN)(4) [M=Rb, Cs, M'=Co, Zn] are isostructural and that these donor arrangements are alpha-type (strictly speaking theta-type). Although the band structure calculation affords the closed Fermi surface due to the large transverse overlap integrals, a metal-insulator (M-I) transition occurs at 20 K for alpha-(BEDT-TTF)(2)CsM'(SCN)(4) [M'=Co, Zn] and at 190 K for alpha-(BEDT-TTF)(2)RbCo(SCN)(4), respectively. The M-I transition temperatures of the former M=Cs salts are elevated by applying pressure. The magnetic susceptibilities of the M'=Co salts obey the Curie-Weiss law with theta=3-6K from 300 K to 20K. The relatively large theta might be originated from the superexchange of an anion network like...Co2+...NCS- as well as interaction through itinerant electrons in BEDT-TTF layer. Below 20 K the anomaly of chi was observed, which can be explained by a crystal field around Co2+.
