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A simple synthesis of the zerovalent complex Mo[P(OCH3)3]6 has been devised from a potassium reduction of MoCI4(tetrahydrofuran)2 followed by reaction with trimethyl phosphite at 70°. Protonation of this octahedral complex gave only low yields of the expected seven-coordinate hydride, HMo[P(OCH3)3]6+. The major product was an octahedral nonhydridic cation, Mo[P(OCH3)3]5P(OCH3)2+, derived from proton cleavage of the P─O phosphite ester bond. This octahedral cation was stereochemically nonrigid, apparently through facile methoxy group migration. Close packing by methoxy groups in this fluxional cation was established through an x-ray crystallographic study of Mo[P(OCH3)3]5P(OCH3)2+- PF6−. Extended reaction of trifluoroacetic acid with Mo- [P(OCH3J. yielded the seven-coordinate hydride, HMo- [P(OCH3)3]4(O2CCF3), which was near pentagonal bipyramidal and was stereochemically nonrigid.