Crystal structures of cis-dibromodioxobis(triphenylphosphine oxide)molybdenum(VI), cis-dichlorodioxobis(triphenylphosphine oxide)molybdenum(VI), and cis-bis(butane-2,3-diolato)dioxomolybdenum(VI)–butane-2,3-diol (1/2): a comparison of co-ordination spheres and the general stereochemistry of molybdenum(VI) oxo-complexes

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Abstract
The crystal and molecular structures of the title complexes cis-[MoO2Br2(PPh3O)2](1), cis-[MoO2Cl2(PPh3O)2](2), and the i.r. spectra of these complexes and of a third complex, cis-[MoO2F2(PPh3O)2](3), have been determined. Crystal data: (1), space group P21/c, Z= 4, a= 19.081(1), b= 9.964(1), c= 19.202(1)Å, β= 111.52(2)°, R= 0.068 for 5 023 reflections; (2), space group P21/c, Z= 4, a= 18.95(3), b= 10.003(1), c= 19.278(2)Å, β= 112.06(4)°, R 0.052 for 6 520 reflections. Complexes (1) and (2) have distorted octahedral symmetry with cis PPh3O ligands. Apart from those bonds involving halogens, there are no significant differences in bond lengths in the two complexes. Assignments of the i.r. spectra of (1)–(3) have been correlated with their structural features. General aspects of the stereochemistry of molybdenum(VI) complexes are highlighted, and it is shown how these can be successfully applied to predict the gross stereochemistry of a molybdenum(VI) complex with butane-2,3-diol (HL), cis-[MoO2L2]·2HL, whose structure was previously unknown. Crystal data for (4): space group C2/c, Z= 4, a= 16.269(5), b= 9.848(5), c= 14.313(5)Å, β= 91.31(5)°, R 0.076 for 1 368 reflections. The crystal contains isolated MoO2L2 molecules connected by hydrogen bonds to two HL molecules. The complex also has distorted octahedral symmetry with two short, two medium, and two long Mo–O bonds. A general principle is established that in mixed-ligand complexes the weaker π-bonding donors are found to be trans to the terminal oxygen where they are not directly competing for the available empty metal 4d orbitals.

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