Structures of +X- Complexes. An Experimental and Theoretical Study of Ring-Whizzing

Article abstract of DOI:10.1021/ja00365a020

Complexes of the formula <(Ph3C3)M(PPh3)2>X, where X = Ni, Pd and X = ClO4, PF6, have been prepared by the reaction of (ethylene)M(PPh3)2 with triphenylcyclopropenium perchlorate or hexafluorophosphate.Complete X-ray analysis nas been carried out for <(Ph3C3)Ni(PPh3)2>PF6 (1), <(Ph3C3)Pd(PPh3)2>ClO4 (2), and <(Ph3C3)Pd(PPh3)2>PF6*C6H6 (3).The crystal data are as follows: (1) a = 15.815 (4) Angstroem, b = 13.781 (4) Angstroem, c = 12.764 (4) Angstroem, α = 114.06 (9) deg, β = 95.92 (9) deg, γ = 97.74 (9) deg, Z = 2, space group P<*>; (2) a = 11.115 (4) Angstroem, b = 35.486 (9) Angstroem, c = 12.584 (4) Angstroem, β = 104.49 (7) deg, Z = 4, space group P2₁/n; (3) a = 12.130 (5) Angstroem, b = 23.863 (7) Angstroem, c = 18.669 (6) Angstroem, β = 91.45 (8) deg, Z = 4, space group P2₁/n.The structures were refined to R values 0.079, 0.079, and 0.065 for 1, 2, and 3, respectively.These three structures, along with a previously determined one where M = Pt, X = PF6, 4, show a progressive movement of the (Ph3P)2M unit over the face of the cyclopropenium cation.In other words, these structures chart the reaction path for going from η² geometry, where the (Ph3P)2M unit is positioned below one carbon-carbon bond, to an equivalent η² geometry.The movement of the (PPh3)2M group is accompanied by rotation, as well as a number of other geometrical changes.A potential surface for this ring-whizzing motion was determined by molecular orbital calculations of the extended Hueckel type.The calculations mimic the geometrical details experimentally found and provide an electronic rationale for the observed distortions.The calculations and observed structures are in agreement with the McIver-Stanton theorem which regulates the symmetry of potential surfaces.The effect of substitution on cyclopropenium ring and of changing the phosphines to other ligands on the potential surface is also discussed.