Dioxidomolybdenum(VI) compound [MoO₂Cl₂(dmso)₂] reacts with a chiral tetradentate O₃N-type l-alanine bisphenol ligand precursor (Et₃NH)H₂L^ala to form an oxidochloridomolybdenum(VI) complex [MoOCl(L^ala)] (1) as two separable geometric isomers with phenolate groups in cis or trans positions. The single crystal X-ray and NMR analyses of cis- and trans-1 reveal that the complexes are formed of monomeric molecules, in which the ligand has a tetradentate coordination through three oxygen donors and one nitrogen donor. The reaction of Na₂MoO₄·2H₂O with the same ligand precursor in an acidic methanol solution leads to the formation of an anionic dioxido complex (Et₃NH)[MoO₂(L^ala)] (2) with a trans coordination of the tetradentate ligand. Trans-1 and 2 were studied as active catalysts for olefin epoxidation: i.e. styrene, cyclohexene, S(–)-limonene and (–)-α-pinene using H₂O₂ and tBuOOH as oxidants.