Abstract Paramagnetic tetrakis(2,6-dimethylphenyliso- cyanide)cobalt(II) perchlorate, [Co(CNR) 4 ](ClO 4 ) 2 , R  2,6-Me 2 C 6 H 3 , has been prepared from pentakis- (2,6-dimethylphenylisocyanide)cobalt(II) perchlorate hemihydrate, [Co(CNR) 5 ](ClO 4 ) 2 ·0.5H 2 O, in novel reactions with triphenylarsine and triphenylstibine. Analogous reaction with [Co(CNR) 5 ](BF 4 ) 2 ·0.5H 2 O produced only [Co(CNR) 5 ]BF 4 . Both [Co(CNR) 5 ]- X 2 ·0.5H 2 O, X  ClO 4 − , BF 4 − , can be recrystallized from CH 2 Cl 2 , CHCl 3 or CH 2 ClCH 2 Cl in 0.15–0.30 M initial concentration, to produce [Co(CNR) 5 ]X 2 ·S (S  adducted solvent) in good yield. Recrystallization of [Co(CNR) 5 ](ClO 4 ) 2 ·0.5H 2 O from dilute (C M ⩽ 5 X 10 −3 M) solution, however, produces [Co(CNR) 4 ](ClO 4 ) 2 ·S, while [Co(CNR) 5 ](BF 4 ) 2 · 0.5H 2 O regenerates the starting material. Recrystallizations from CH 3 CN or CH 3 NO 2 produce [Co(CNR) 5 ](ClO 4 ) 2 ·0.5H 2 O (concentrated solution) or no recovered product (dilute solution). Solid-state properties of magnetic susceptibilities, diffuse reflectance electronic spectra, and mull-infrared are investigated. Distorted square planar coordination of RNC in [Co(CNR) 4 ](ClO 4 ) 2 is suggested. Infrared v (ClO 4 ) strongly suggests monodentate ClO 4 − coordination in [Co(CNR) 4 ](ClO 4 ) 2 , or [Co(CNR) 4 (ClO 4 ) 2 ], while the possibility of weaker ClO 4 − coordination in [Co(CNR) 5 ](ClO 4 ) 2 ·0.5H 2 O is ambiguous. Isolation of [Co(CNR) 4 ](ClO 4 ) 2 is the first observed incident of significant difference between BF 4 − and ClO 4 − salts of Co(II)-arylisocyanide complexes.