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)

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 conformation...

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