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What is the Real Steric Impact of Triphenylphosphite? Solid-State and Solution Structural Studies of cis- and trans-Isomers of M(CO)4[P(OPh)3]2 (M = Mo and W)

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posted on 2007-12-31, 00:00 authored by Donald J. Darensbourg, Jeremy R. Andreatta, Sarah M. Stranahan, Joseph H. Reibenspies
The steric requirements for the triphenylphosphite ligand in several molybdenum and tungsten carbonyl derivatives have been shown by X-ray crystallography to exceed the original Tolman’s cone angle of 128°. That is, due to various accessible conformers possible for P(OPh)3, solid-state data predict a considerably larger cone angle for the ligand of between 140° and 160°. Importantly, the solution behavior of cis-M(CO)4[P(OPh)3]2 (M = Mo or W), coupled with similarly reported observations on a series of cis-Mo(CO)4[PR3]2 derivatives, support this conclusion, for these molecules both undergo thermal rearrangement to the more stable trans-isomers. On the other hand, the electronically similar but sterically much smaller cis-Mo(CO)4[P(OCH2)3CEt]2 complex is thermally stable under much harsher conditions. Furthermore, a comprehensive survey of structural data for transition-metal−triphenylphosphite derivatives available in the Cambridge Crystallographic Database reveals that most molecules display conformations that dictate cone angles much greater than that originally suggested by Tolman.

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