Terminal Acetylenes React to Increase Unsaturation in [(<sup>t</sup>Bu<sub>2</sub>PCH<sub>2</sub>SiMe<sub>2</sub>)<sub>2</sub>N]Re(H)<sub>4</sub>

(PNP<sup>tBu</sup>)Re(H)<sub>4</sub>, where PNP<sup>tBu</sup> is (<sup>t</sup>Bu<sub>2</sub>PCH<sub>2</sub>SiMe<sub>2</sub>)<sub>2</sub>N, reacts at 23 °C with RC⋮CH (R = <sup>t</sup>Bu, SiMe<sub>3</sub>, Ph) to give first H<sub>2</sub> and mirror-symmetric (PNP<sup>tBu</sup>)ReH<sub>3</sub>(CCR), then H<sub>2</sub> and <i>C</i><sub>2</sub><i><sub>ν</sub></i> symmetric (PNP<sup>tBu</sup>)Re(CCR)<sub>2</sub>. The diacetylide compounds show temperature-independent paramagnetism and <sup>13</sup>C and <sup>31</sup>P chemical shifts far beyond their normal values for other (PNP<sup>tBu</sup>)ReX<i><sub>n</sub></i> compounds. Single-crystal X-ray diffraction shows very similar structures for the cases R = Ph and R = SiMe<sub>3</sub>, each having an approximately <i>C</i><sub>2</sub><i><sub>v</sub></i> geometry with equivalent acetylides with ∠C−Re−C approximately 108°. No hydride or H<sub>2</sub> ligands are detected in final difference Fourier maps. DFT(B3PW91) calculations give minimum energy geometries of these species, of their products upon adding H<sub>2</sub>, and of mechanistically significant analogues [(H<sub>2</sub>PCH<sub>2</sub>SiH<sub>2</sub>)<sub>2</sub>N]ReH<i><sub>n</sub></i>R‘<i><sub>m</sub></i>H<sub>2</sub><sub>-</sub><i><sub>m</sub></i>, with <i>n</i> = 0, 2, <i>m</i> = 1, 2, and R‘ = H or Ph. These calculated geometries, when compared to those from X-ray diffraction, indicate that the isolated compounds have no hydride or H<sub>2</sub> ligands and are thus (PNP)Re<sup>III</sup>(CCR)<sub>2</sub>, making them more unsaturated than the reagent (PNP)Re<sup>V</sup>(H)<sub>4</sub> by two electrons. Triplet state geometries of (PNP)ReXY are calculated and analyzed, as are their frontier orbitals.