Introduction. -da ML, fragments play an important role in organo-transition-metal chemistry. They act either as monovalent molecular units displaying a triplet or a singlet electronic state. This feature has a great impact on the chemistry of such species [l]. On the other hand, d8 ML, moieties can provide bivalency under oxidative conditions which normally leads to d6 octahedral systems. Thus, these fragments demonstrate chemical versatility, which is also reflected in their frequent occurrence in cycles of homogeneous catalysis that often require this property.The actual reactive character of such unsaturated species is dependent on the nature of the ligands and the transition-metal center. Tuning of both of these factors would allow proper adjustment to bring about a desired type of chemistry. Following this idea, in earlier papers we have extensively investigated the chemistry of [Fe(CO),L,] fragments [2]. In continuation of this work and to widen the scope of ligand and metal influence in L,M species, we set out to examine the synthetic access to and the reactivity of isoelectronic complexes with [Re(CO)(NO)LJ moieties (L = phosphorus donor), which have, up to now, only scarcely been subjected to thorough studies [3].

Syntheses of disubstituted, tetramethylethanediyl-bridged titanocene derivatives (CH3)4C2(1-CsHr3-RhTiCI2' with R = t-butyl, trimethylsilyl, isopropyl, a,a-dimethylbenzyl and I-phenylcyc1ohexyl, by reductive coupling of the appropriately substituted 6,6-dimethylpentafulvenes with Mg/CCl 4 and reaction of the resulting di-Grignard compounds with TiC1 3-3THF are described along with those of several zirconium analogues. The reaction sequence in each case gives, together with the desired racemate, comparable amounts of the corresponding meso-configurated complex. The NMR data for the racemic and meso diastereomers of these compounds and crystal structures of one meso and several racemic representatives are reported.

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