The crystal and molecular structure of the molybdenum tetracarbonyl complex of 1,4-diphenyl-2,2′,3,3′5,5′,6,6′-octamethylcyclo-1,4- diphospha-2,3,5,6-tetrasilahexane

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Abstract
The crystal and molecular structure of the molybdenum tetracarbonyl complex of 1,4-diphenyl-2,2′,3,3′,5,5′,6,6′-octamethylcyclo-1,4-diphosph a-2,3,5,6-tetrasilahexane, (PhPSi2Me4)2Mo(CO)4, has been determined by single crystal X-ray diffraction. The crystals are monoclinic, space group P21/n, with a = 15.983(5), b = 19.316(6), c = 10.580(3) Å, β = 99.99(2)° V = 3217(2) Å3, Z = 4, and ρcalcd = 1.356 g/cm3. The structure was solved by Patterson heavy atom methods and was refined by full-matrix least-squares procedures to a final R1 of 3.3% and R2 of 4.3%, for 6342 reflections with intensities greater than 2σ. The coordination of the boat-shaped (PhPSi2Me4)2 ligand to molybdenum tetracarbonyl produces a distorted octahedral environment about the metal. The mean Si—Si (2.358(14) Å) and P—Si (2.275(19) Å) distances are not greatly altered from the values found for them in the free ligand. The structural consequences of P → Si π-bonding in the latter are therefore minimal. The relatively long P—Mo (mean length 2.592(13) Å) distances are interpreted as the result of reduced back-bonding from the metal, caused by the presence of the relatively electropositive silyl groups on phosphorus. Consistently, the equatorial Mo—C bonds, which are trans to the diphosphine ligand, are shorter (mean length 1.968(19) Å) than the corresponding bonds to the axial carbonyl groups (mean length 2.021(15) Å).

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