Metal-metal bonding has inspired scientists for many dec-A C H T U N G T R E N N U N G ades [1] and a lot of this fascination and interest has recently shifted towards unsupported metal-metal linkages.[2-4] Unsupported REÀTM bonds (RE = rare earth, TM = transition metal) are rare [5][6][7] and especially important with regard to the general understanding of RE À TM bonding. RE À TM bonding determines, for instance, the nature/characteristics of the corresponding intermetallic compounds of which many play an important role in our daily life. RE À TM intermetallic compounds are in high-performance permanent magnets, [8,9] hydrogen-storage materials [10] and car batteries.[11] Furthermore, REÀTM bonds are strongly polar [6,12] and mixed-metal entities containing metal atoms (or cluster subunits) with significantly different electronic properties offer the potential to meet chemical challenges like the rational design of higher aggregated systems.[13] The expected reactivity behaviour results from the introduction of metalmetal bond polarity. It is similar to synthetic strategies employed in organic chemistry, in which rational design of complicated functionalised hydrocarbons can be based on specifically introduced bond polarities; for example, the generation of nucleophilic or electrophilic centres of reactivity for the regio-or even stereoselective formation or cleavage of C À C bonds. [14] It may allow us to synthesise, in a controlled fashion, monodisperse aggregates of increasing size starting from simple bimetallic complexes and small clusters to nanosized materials and even bulk metals in order to get specific access to the plenitude of physicochemical effects along this path.We report here on the synthesis, the crystal structure and the electronic structure of molecular [Yb(TM) 2 ] intermetalloids [15] in which divalent ytterbium displays two unsupported bonds with either two rhenium or two ruthenium atoms.RE À TM bonding in coordination compounds of low valent lanthanoids is connected with the name of the Shore group, [16] who synthesised Yb II À Fe coordination polymers. Inspired by this pioneering work we decided to use an ytterbium(II) ion as the RE "partner". In addition to the complications one has in terms of forming unsupported REÀTM bonds in general, [17] the instability of the low oxidation state of the lanthanoid has to be addressed. In order to avoid single-electron transfer from the divalent RE atom to the transition metal we chose [CpRu(CO) 2 ]À as the TM "partner". The oxidation state of Ru is zero and it is unlikely to be further reduced.

Lithium bis(trimethylsilyl)amide, LiN(SiMe3)2, reacts with Cp*V(O)Cl2 and Cp*TaCl4 to give trimethylsilylimido complexes such as [Cp*V(NSiMe3)(μ‐NSiMe3)]2 (7) and Cp*Ta(Cl)(NSiMe3)[N(SiMe3)2] (19), respectively. Substitution of the chloro ligand in 19 by anionic groups leads to complexes with 3 different N‐containing ligands, Cp*Ta(X)(NSiMe3)[N(SiMe3)2] (X = N3 (20) or NPEt3 (21)). Complex 7 is air‐ and moisture‐sensitive, and several derivatives containing oxo and trimethylsiloxy ligands have been identified. Trimethylsilyl azide, Me3Si‐N3, is able to replace the oxygen‐containing ligands for azido ligands. The two complete series of bis(azido)‐bridged complexes, [Cp*VCln(N3)2‐n(μ‐N3)]2 (n = 2, 1, 0) and [Cp*TaCln(N3)3‐n(μ‐N3)]2(n = 3, 2, 1, 0), are accessible from the reactions of Cp*VCl3 and Cp*TaCl4, respectively, with trimethylsilyl azide. A bis(nitrido)‐bridged azido‐vanadium complex, [Cp*V(N3)(μ‐N)]2 (18), has also been obtained and structurally characterized.

The reaction of Cp*V(CO)4 (1) with molecular oxygen in diluted pentane solution leads to a tetrameric dioxide, [Cp*V(O)(μ-O)]4 (2), which is a precursor of the octanuclear aggregate Cp*6V8O17 (3). The molecular structures of 1 and 2 have been determined by X-ray crystallography.

Source of materialCrCl 3 (thf) 3 (0.43 g, 1.15 mmol), KAp* (0.57 g, 1.15 mmol) (Ap* = deprotonated (2,6-diisopropyl-phenyl)-[6-(2,4,6-triisopropylphenyl)-pyridin-2-yl]-amine) [1] and THF(30 mL)were stirred for 16 hinaSchlenk flask. The solventwasremoved under vacuum and amixture of n -pentane/ether (60 mL;1:1, v / v )wasadded. The suspension wasfiltered and upon standing at-80°Cfor five days, green crystals of the title complex were formed. Experimental detailsThe Hatoms attached to Natoms were included in the refinement process due to their importance with regard to the H-bond pattern. DiscussionThe title compound is adinuclearc hromium(IV)c omplex. Each of the chromium atoms adopts anoctahedralcoordination. They are coordinated by two chloro ligands and connected via a m -chloro and two m -oxygen bridges with bond angles of

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