Terminal Acetylenes React to Increase Unsaturation in [(^tBu₂PCH₂SiMe₂)₂N]Re(H)₄

(PNP^tBu)Re(H)₄, where PNP^tBu is (^tBu₂PCH₂SiMe₂)₂N, reacts at 23 °C with RC⋮CH (R = ^tBu, SiMe₃, Ph) to give first H₂ and mirror-symmetric (PNP^tBu)ReH₃(CCR), then H₂ and C_2ν symmetric (PNP^tBu)Re(CCR)₂. The diacetylide compounds show temperature-independent paramagnetism and ¹³C and ³¹P chemical shifts far beyond their normal values for other (PNP^tBu)ReX_n compounds. Single-crystal X-ray diffraction shows very similar structures for the cases R = Ph and R = SiMe₃, each having an approximately C_2v geometry with equivalent acetylides with ∠C−Re−C approximately 108°. No hydride or H₂ ligands are detected in final difference Fourier maps. DFT(B3PW91) calculations give minimum energy geometries of these species, of their products upon adding H₂, and of mechanistically significant analogues [(H₂PCH₂SiH₂)₂N]ReH_nR‘_mH_2-m, with n = 0, 2, m = 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 H₂ ligands and are thus (PNP)Re^III(CCR)₂, making them more unsaturated than the reagent (PNP)Re^V(H)₄ by two electrons. Triplet state geometries of (PNP)ReXY are calculated and analyzed, as are their frontier orbitals.