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The potential energy surfaces of protonated N2, P2, and PN are explored at RHF, MP2, and CISD levels. Stationary structures were not only optimized but also characterized by computation of their vibrational frequencies at each of these levels. Model dependencies of potential energy surface characteristics, geometries, vibrational frequencies, relative isomer stabilities, and proton affinities are studied in a systematic fashion. The potential energy surface of HP2+ exhibits dramatic model dependencies and it is clarified by a scan of the potential energy surface as a function of the H-P-P angle at the CZSD level. Geometries and vibrational frequencies of the most stable isomers also are reported at the CISD(full)/6-3llG(df,p) level. Accurate scale factors for bond lengths and vibrational frequencies determined for the neutral diatomics X s X can be applied successfully to the protonated species as well. End-on protonation of Nz, edge-on protonation of P2, and N-protonation of PN are favored, and our best estimates for the proton affinities are 116.3, 161.2, and 194.2 kcal/mol, respectively.Proton affinities parallel the increase of the polarizabilities (N2 < N P < P2) but the polarizabilities perpendicular to the bond axis, ape,.,,, are significantly small than apra and their ratio apra/aperp is not related to the isomer preference energies. For protonated N2 and P2, the isomer preference is closely related to the X2 bond length per se and the structural preferences of the protonated systems thus reflect the same factors that also determine the X=X bond lengths in the neutral diatomics. The proton is atypically electrophilic compared to carbenium ions and the carbenium ion affinities of N2, P2, and PN are generally much smaller than proton affinities. While the potential energy surfaces of protonated and methylated Nz and PN are qualitatively similar, the respective derivatives of P2 differ significantly.

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Structural effects of the nature of the phosphines on trans-tricarbonylbis(phosphine)iron(0) complexes trans-Fe(CO)3L2 (L = PPh2Me, PPh3) are explored. The X-ray crystal structure of trans-Fe(CO)3(PPh2Me)2, 1, is reported. Complex 1 crystallizes in the monoclinic space group P21/c with cell parameters a = 15.551(6) Å, b = 9.7024(12) Å, c = 17.320(6) Å, β = 91.081(11), and Z = 4. Refinement resulted in R = 0.045 and R w = 0.054 for 3614 independent reflections with I > 2.0σ(I). The structure of 1 is compared to trans-Fe(CO)3(PPh3)2, 2, and its etherate 2·OEt2, 3. Further comparisons are made to the isoelectronic complex trans-[Co(CO)3(PPh3)2]+, 4, and the radical cation [Fe(CO)3(PPh3)2]•+, 2 • + . Phosphine replacement affects intrinsic structural properties of 1−3 only marginally. In particular, the average Fe−P bond lengths in 1 (2.206 Å), 2 (2.217 Å), and 3 (2.216 Å) all are essentially the same in light of the standard deviations. The structures of the free and complexed phosphines also are essentially identical. Complexes 1−3 show a common motif of distortion from the trigonal planar (pseudo) C 3 v bipyramid (P−Fe−P nonlinearity, Fe(CO)3 C 2 v -distortion, and phosphine nonequivalence). Ab initio calculations for trans-Fe(CO)3(PH3)2 at the MP2/LANL1DZ level suggest that these distortions are due to packing. The phosphines are more or less staggered with respect to the carbonyls and the methyl groups are gauche. Co(I) versus Fe(0) replacement retains the trigonal bipyramidal structure while the oxidation of 2 to 2 • + yields the square pyramidal structure with longer trans Fe−P bonds (2.282 Å). Of special interest are the unexpected phosphine conformations in 1−3 and they are shown to be beneficial for the optimization of intermolecular arene−arene interactions. The crystal packing of 1 reveals displaced face-to-face and displaced T-shape arene−arene contacts that place the two phenyl rings in relative orientations that lead to stabilization in the respective benzene dimers. A rotated displaced T-shape arrangement plays a crucial role in 2. The analyses emphasize the interplay between intermolecular arene−arene packing interactions and the phosphine conformations.

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