[Mo(eta3)-C3H5)(CO)2(bipy*)Cl] undergoes trigonal twist rearrangements in solution, so that three isomers are coexisting. It was used as a starting material leading to a dinuclear complex containing a hydrogen-bonded network of H(2)O and crown-ether molecules between two Mo(eta3-C3H5)(CO)2(bipy*) moieties.

In order to generate 2‐propenylphenolate, PO−, as a potential ligand for oxomolybdenum compounds, the lithiation of the parent phenol derivative, POH, was investigated. Treatment of POH with BuLi in THF yielded a tetramer of the corresponding alkoxide “POLi” with a heterocubane structure in which the coordination spheres of the Li centres are completed by THF molecules, i.e. {Li(THF)[μ3‐OC6H4(CH=CH−CH3)‐2]}4 (1). Compound 1 proved to be rather unreactive and unsuitable as a starting material for the introduction of the corresponding aryloxide ligands into the coordination sphere of molybdenum by salt metathesis. The latter did occur, however, when “POLi” was synthesised in the presence of 12‐crown‐4 and subsequently treated with molybdenyl chloride. This led to an unidentified aryloxide compound as well as to a hitherto unknown chlorooxomolybdate anion in the complexes [Li(12‐crown‐4)][MoO2Cl3(THF)] (2) and [{Li(12‐crown‐4)}2Cl][MoO2Cl3(THF)] (2′) with an octahedral coordination sphere at the molybdenum centre in each case. An Mo(PO) complex was finally isolated for the first time when MoO2(OtBu)2 was employed as a starting material and treated with “POLi” prepared in situ in the presence of the crown ether. This led to the isolation of [Li(12‐crown‐4)2][MoO2(PO)3] (3), which represents a rare example of a five‐coordinate dioxomolybdenum(VI) complex as well as being the first example among these complexes with a square‐pyramidal ligand arrangement. Comparison of 2 with 3 suggests that the low coordination number of the 14‐electron compound 3 originates from the bulkiness of the PO− ligands. The crystal structures of all three compounds 1−3 are discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Photolysis of [CpMo(CO)3Cl] in the presence of 2-(1-propenyl)phenyldiphenylphosphane (PP, cis/trans mixture) yields the complex [CpMo(CO)Cl(PP)], 2, with an olefin ligand tethered to a phosphorus donor atom. A trans configuration of the substituents at the olefinic unit is thermodynamically preferred, so that the corresponding complex 2 t can be isolated in isomerically pure form under certain conditions. However, the cis-olefin complex 2 c has been characterized in its mixture with 2 t , too. Treatment of 2 t with HNMe2 leads to the intramolecular elimination of HCl under formation of the compound [CpMo(CO)(CH2CHCHC6H4PPh2)], 3, bearing a chelating π-allyl/PPh2 ligand. When a CH2Cl2 solution of 3 is photolyzed in the presence of N2O, the CO ligand is replaced by two chloride ligands, i.e., the Mo center is oxidized to the oxidation state +IV. The resulting complex [CpMoCl2(CH2CHCHC6H4PPh2)], 4, is one of the rare examples, where a soft allyl ligand is bound to a metal center in a comparatively high oxidation state. However, probably due to the chelate effect, the Mo−allyl bond is remarkably stable. All compounds 2−4 have been investigated by means of single-crystal X-ray diffraction, and their crystal structures are discussed.

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