A number of new adducts of the metal-halogen cluster Mo₆Cl₈⁴⁺ have been prepared and characterized. Monodentate ligands generally yield complexes of the type [(Mo₆Cl₈)Cl₄2L]; these compounds show only low ionization in donor solvents and are amorphous to X-rays. On the other hand, bidentate ligands form crystalline derivatives of composition [(Mo₆Cl₈)Cl₄(L)₂]Cl₂ or [(Mo₆Cl₈)Cl₃L]Cl. The noteworthy features of these adducts is the characteristic six-fold coordination to the cluster, irrespective

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... uster, irrespective of ligand type. It was found that rhenium trichloride and tribromide had similar visible absorbtion spectra to the Re₃Cl₁₂³- anion and presumably comparable trinuclear structures, Re₃x₉. This was substantiated by the isolation of the halogene-anions Re₃Cl₁₁²-, Re₃Br₁₂³-, Re₃Br₁₁²- and Re₃Br₁₀- from these tribaludes. The composition of these anions can be correlated with the cation size and charge, and their formation suggests that the terminal in plane halogen atoms are extremely labile, Radiochemical exchange and reaction with thiocyanate ion confirmed this deduction and demonstrated that the six out of plane halogen atoms also are relatively labile, as compared with the kinetically inert bridging halogens. Thus the basic unit which determines the chemical and physical properties if these clusters is Re₃x₃. The niobium and tantalum polynuclear halides, Nb₆Cl₁₄7H₂0 and Ta₆Cl₁₄7H₂0, do not coordinate as easily as the cluster Mo₆Cl₈⁴⁺ with monodentate and bidentate ligand, and the products which were obtained, namely M₆Cl₁₄(pyr)₂ and M₆Cl₁₄(pyr-N-oxide) were less amenable to study. Finally, the common bonding, Structural and spectral features of these metal-halogen clusters have been discussed.