A series of cobalt(III) complexes containing 2-cyanoethylphosphines, [Co(Me(2)dtc)(2){P(CH2CH2CN)(n)Ph3-n}(2)]BF4 {Me(2)dtc(-): N,N-dimethyldithiocarbamate; n =1 (1), 2 (2), and 3 (3)} have been prepared, and their molecular structures in the crystals and in solution have been characterized. X-ray analysis revealed that all complexes isolated as single-crystals were the trans-isomers. The Co-P bond lengths for the 2-cyanoethylphosphines in complexes 1-3 are almost invariant to the number of 2-cyanoethyl substituent groups. Unlike related complexes with other monodentate P-donor ligands, complexes 1-3 in solution showed dissociation and isomerization equilibriums, which were achieved immediately after dissolving the crystals. These characteristic features of the complexes would result from the weak sigma-donor and pi-acceptor abilities of 2-cyanoethylphosphines. Furthermore, mixing of complex 1 and NaN3 in acetonitrile gave cis-[Co(Me(2)dtc)(2)(N-3){P(CH2CH2CN)Ph-2}] quantitatively, and refluxing the mixture resulted in the phosphine-tetrazolate complex, [Co(Me(2)dtc)(2)(Ph2PCH2CH2CN4-(KP)-P-2,N-1)] (4).