The Structure of an Iodide Complex of the Type Mo 2X 4(LL) 2 Exhibiting Two Different Rotational Geometries in the Solid State

Abstract

The compound which we have prepared by reacting Mo 2(μ-dppm) 2Cl 4 with NaI in acetone and which has a stoichiometry dose to that for Mo 2-(dppm) 2I 4, forms thin plates when crystals are grown from CH 2Cl 2CH 3OH. It crystallizes in the triclinic space group P 1 with the following unit cell dimensions: a = 14.931(6), b = 22.960(7), c = 12.794(4) Å, α = 100.30(2), β = 114.76(3), γ = 93.90(3)°, V = 3868(6) Å 3, and Z = 3. The structure was refined to R = 0.088 ( R W = 0.125) for 6430 data with F 2> 3.0σ( F 2). There are two independent sets of molecules in the unit cell both of which possess a structure in which the dppm ligands are present in an intramolecular bridging mode with a transoid disposition to one another. One molecule, which resides on a crystallographic inversion center, has an apparent MoMo distance of 2.178(3) Å and an eclipsed rotational geometry, while the two symmetry related molecules in general positions have a MoMo distance of 2.152(2) Å, and a twist angle ( i.e. average torsional angle, X) of 17 [2] Å. This is one of the first instances where two quite different rotational geometries are present in the solid state for a molecule of this general type, a result which indicates that crystal packing forces can play an important role in determining the exact rotational geometry. There is crystallographic and chemical evidence that these crystals contain small amounts of chloride and that the true composition may actually be Mo 2(dppm) 2- I 4− x Cl x , with x∼0.2.