Abstract The pseudohalogens (XCN)2 (X = S or Se) react with n-C5H5Co(CO)2 and n-Me3SiC5H4Co(CO)2 with formation of the 18 electron complexes, RCo(CO)(XCN)2 (R = n-C5H5 or n-Me3SiC5H4). By reactions with X(CN)2 (X = S or Se), the cyclopentadienyl cobalt dicarbonyls are transformed into the mixed complexes, RCo(CO)(CN)(XVN) (R = n-C5H5 or n-Me3SiC5H4. All the compounds are characterized by elemental analysis, IR, electronic and 1H NMR spectra. The thiocyanogen derivatives undergo linkage isomerism (N→S) in polar solvents.