Pentakis (phenyl- and para-substituted phenylisocyanide)cobalt(I) has been re-crystallized from various solvent types to study further the five-coordinate conformational isomerism observed for [Co(CNC 6H 5) 5]ClO 4 re-crystallized from chlorohydrocarbons. Characterization is primarily through the −NC i.r. pattern and proton-NMR. Perchlorate and tetrafluoroborate salts are used. The aromatic isocyanides include C 6H 5NC, p-FC 6H 4NC, p-ClC 6H 4NC, p-BrC 6H 4NC, p-IC 6H 4NC, and p-CH 3C 6H 4NC. Three isomeric structures are maintained with phenylisocyanide but only one geometry is observed with para-substituted phenylisocyanides.