The discussion of mixed-valence (MV) or intervalence (IT) transition-metal complexes has been largely restricted to systems containing a reducing and an oxidizing metal center." If the metal-metal interaction is weak, definite oxidation states can be assigned to the metals and charge-transfer (CT) or IT electronic transitions from the reduced to the oxidized metal occur at relatively low energies. Mononuclear metal complexes in which a ligand occurs in different oxidation states (L,ML,,) are also MV compound^.^.^ However, in the electronic spectra of complexes containing catecholate and o-semiquinone as MV ligands, IT absorptions were not detectedS6 MV ligands seem also to be present in the 1,2-dithiolene complexes of Ni(II), Pd(II), and Pt(II)7 (A). Thes' A electronic structure is described by two equivalent resonance forms.* The highest occupied (2b1,) and the lowest unoccupied (3b2.J ligand orbitals are equally delocalized over both The 2bl, -3bze electronic transition has then no CT contribution. However, one of both resonance forms may prevail in the ground state if the complex becomes unsymmetric by the proper choice of substituents at the ligands." The ground state is represented almost exclusively by one of both mesomeric forms, when the 1,2-dithioketone is replaced by 1,2-diimine as oxidizing ligand12 (B). The 2bl, -3bzp(in DZh symmetry) transition becomes now a C T or IT transition from the reducing 1,2-ethylenedithiolate to the oxidizing 1,2-diimine ligand.12,13 The present investigation was undertaken to gain more insight into the electronic structure of these ligand-based MV complexes with particular reference to the influence of the metal. Results and DiscussionSynthesis. Six complexes of the type M( 1,2-diimine)-(ethylene-1,2-dithiolate) (111) were prepared with nickel,

Details of the crystal structure investigation can be obtained from the Fachinformationszentrum Energie Physik Mathematik, D-7514 Eggenstein-Leopoldshafen by quoting the depository number CSD 50259, the name of the author and the citation of the journal.

Fig. I. Struktur von [Fer(CO)6(PrrBu2)] 2 im Kristall 141. Abstlnde: Fe-Fe 274.0(1), P-P 205.9(3), Fe-P 221.2-223.2(2) pm.

Die Darstellungen der Komplexe (I), die zwei unterschiedliche, einen reduzierenden und einen oxidierenden, Liganden enthalten, erfolgen ausgehend von Dichloro‐bis‐[biacetyl‐bis‐(anil)]‐Ni(ll), Dichloro‐bis‐[benzonitril]‐Pd oder Na‐tetrachloroplatinat und sind detailliert beschrieben.

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