The crystal structure of (q5-C,H,),W,(CO), has been determined and that of its molybdenum analog redetermined with increased accuracy. The W-W distance (3.222 (1) A) is slightly shorter than the Mo-Mo distance (3.235 (1) A), in continuation of the trend already found from the chromium to the molybdenum compound. The free energy of activation for (presumably rotational) interconversion of the gauche and anti rotamers of the tungsten compound in solution has been evaluated from proton nmr spectra over a temperature range (-20 to +60"); it is slightly higher than that in the molybdenum compound. Possible reasons for the observed trends, namely, Cr >> Mo > W for the M-M distances and Cr < Mo 4 W for the free energies of activation of gauche-anti interconversion, are discussed. The crystallographic data for (q5-C,Hs),-Mo,(CO), are essentially identical with tnose reported previously by Wilson and Shoemaker; the present work merely gives the enhanced precision needed to reveal clearly the difference between d(Mo-Mo) and dgV-W). For (qS-C,H,),W,(CO), the crystallographic data are space group P2,/c, a = 10.374 (1) A, b = 7.971 (1) A, c = 12.047 (2) A, p = 125.83 (l)D, and z = 2 . AIC30798U
ring. Since most of the known silver-aromatic interactions are asymmetric with the shorter Ag-C distance invariant at 2.47 A, it appears that a back-donation interaction is generally of lesser importance.In the case of naphthalene!AgClO-i)) • 4H20 there is one large and new perturbing effect which has been added to the electronic environment of the system. It may be that the water of hydration has altered the electronic requirements of the silver ion such that the hydrated silver ion is no longer as good an electron acceptor and may be considered to be a weaker Lewis acid than the free silver ion. Hence, back-donation in this case becomes more important relative to the overall Ag--interaction. It might be more logical to consider the 2.52 (1) A distance from the silver to the midpoint of the nearest C-C bond as a more accurate measure of the Ag-7 interaction rather than the Ag-C distance of 2.61 (1) A.It is interesting to note that for anthracene(AgC10<)4 • H2013 each water of hydration is associated with two silver ions and relatively little perturbing effect upon the "normal" asymmetric Ag-C interaction is observed.
A mechanistic analysis of the rearrangements and structural investigation of a series of p3-allyl molybdenum and tungsten dicarbonyl bisphosphine complexes are presented. A three-dimensional crystallographic analysis revealed the structure of (p3-C3H5)Mo(CO)2(diphos)Cl to be unlike that of the analogous diether and diamine chelates. Analysis of variabletemperature *H, 13C, and 3IP NMR spectra have allowed the elucidation of the dynamic properties of these complexes. A trigonal twist rearrangement is proposed for the dynamic behavior of these complexes in which a rotation of the triangular face formed by the halogen and two phosphorus atoms relative to the face formed by the aliyl and two carbonyl groups is involved. Barriers for this process are between 10 and 12 kcal/mol in the iodide derivatives. The twist process interconverts certain pairs of isomers while retaining particular elements of chirality, indicating that when complexes of this type rearrange a pathway for racemization may not necessarily be present.
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