Investigations on several tetrakis(arylisonitrile)-metal complexes of general stoichiometry M(CNR) 4 n+ X n - with M = Rh(I), Rh(III), [n =1], [n = 3] or Co(II), [n = 2], R = p-tolyl, p-N,N-dimethylaminophenyl, p-nitrophenyl, e. g. and X - = iodide, ClO 4 - , [B(C 6 H 5 ) 4 ] - e. g. are described. The paramagnetic complexes of stoichiometry Co(CNR) 4 I 2 dimerize in organic solvents to yield dinuclear diamagnetic cations with linear iodide bridges. The amount of association depends on the iodide content and on experimental parameters like temperature and/or concentration and can be followed by IR, ESR and/or NMR techniques. Treating of these compounds with ions X - = ClO 4 - or [B(C 6 H 5 ) 4 ]- gives the dinuclear and diamagnetic complexes of stoichiometry [I-Co(CNR) 4 -I-Co(CNR) 4 I]X. The corresponding isonitrile compounds of rhodium(I) associate in solution to yield linear stacks in the crystals. The solid state properties depend strongly on the type of ligand and the counter anion and vary considerably with the conditions of crystallization. The rhodium(I) species are able to react with the corresponding tetrakis(arylisonitrile)-rhodium(III) complexes to mixed valence solids.