Isolation and Characterization of the Molybdenum Alkylidyne Complex [(F<sub>3</sub>C)Me<sub>2</sub>CO]<sub>2</sub>Mo(C-<i>t</i>-Bu)[N(Ar)PC(H)(CMe<sub>2</sub>Ph)] and Its Conversion to a Phosphamolybdacyclobutene

Novel molybdenum alkylidyne [(F<sub>3</sub>C)Me<sub>2</sub>CO]<sub>2</sub>Mo(C-<i>t</i>-Bu)[N(Ar)PC(H)(CMe<sub>2</sub>Ph)] (Ar = 2,6-(<i>i</i>-Pr)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>; <b>3b</b>) is formed in the reaction of <i>tert</i>-butylphosphaalkyne <b>2</b> with molybdenum alkylidene <b>1b</b>. Alkylidyne <b>3b</b>, which contains a formal (phosphaalkenyl)amido ligand, rearranges quantitatively at elevated temperature to form the alkoxide-shifted phosphamolybdacyclobutene <b>4b</b>. Complexes <b>3b</b> and <b>4b</b> have been fully characterized by spectroscopic and X-ray crystallographic analysis; cleavage of the P⋮C-<i>t</i>-Bu triple bond in complex <b>3b</b> is confirmed.