Preparation and properties of tetrahedro-tetraphosphorus complexes of rhodium and iridium. Molecular and electronic structure of [RhCl(η2-P4)(PPh3)2]

Alvin P. Ginsberg, W. Edward Lindsell, Kevin J. McCullough, Charles R. Sprinkle, Alan J. Welch

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White phosphorus dissolved in dichloromethane or diethyl ether at -78°C reacts with the Rh(I) or Ir(I) complexes [MXL3] (M = Rh, X = Cl, Br, I, L = PPh3; M = Rh, X = Cl, L = P(p-tol)3, P(m-tol)3, AsPh3; M = Ir, X = Cl, L = PPh3) to form yellow or orange tetrahedron-tetraphosphorus complexes [MX(P4)L2]. 31P NMR spectroscopy of [RhX(P4)(PPh3)2] in CD2Cl2 at low temperatures shows the P4 ligand to be ?2-coordinated and deshielded by ca. 240 ppm relative to free P4. The P4 units act as A2B2 spin systems coupling to two 31P nuclei of PPh3 ligands (X2) and to 103Rh(I = 0.5) to give an overall A2B2MX2 spin system. The vibrational frequencies of the P4 molecule in the rhodium complexes have been identified by infrared and Raman spectroscopy and are found to be from 15 to 90 cm-1 lower in energy than the corresponding frequencies in free P4. An X-ray structure determination on [RhCl(P4)(PPh3)2]·2CH2Cl 2 at 185 K shows the crystals to be triclinic, space group P1, with a = 11.853 (2) Å, b = 12.568 (8) Å, c = 14.505 (2) Å, a = 104.41 (4)°, ß = 103.42 (13)°, ? = 84.22 (4)°, V = 2033.5 (19) Å3, Do = 1.58 g cm-3, Z = 2, and Dc = 1.562 g cm-3. The P4 molecule is ?2-bonded to the rhodium atom (mean Rh-P = 2.293 Å) with the metal-bonded P-P edge standing perpendicular to the remaining coordination plane of the metal. The phosphine ligands are bent away from the tetraphosphorus group toward the chlorine (?Ph3P-Rh-PPh3 = 166.10 (5)°). The bonded edge of the P4 molecule (P-P = 2.4616 (22) Å) is lengthened by ca. 0.25 Å compared to the edge of a free P4 molecule; the nonbonded edges are essentially unchanged from the free molecule. EHMO and SCF-Xa-SW calculations on [RhCl(P4)(PH3)2] support the analogy between ?2-bonded P4 and ?2-bonded alkene or S2; the "back-bonding" component may be traced to a three-orbital-four-electron interaction between P4 and the RhCl(PH3)2 fragment. The Xa calculations show that the most important contribution to the Rh-P4 covalent bond comes from an equatorial in-plane p overlap of Rh 4dyz with a P4 2Pp*? orbital. There is also a contribution from s overlap of an Rh (4dz2, 4dx2-y2, 5s) hybrid orbital with a P4 2P(Ps, p?, ss) hybrid. The calculated P-P bond order is 0.4 for the bonded edge and 1.0 for the opposite tetrahedral edge of the P4 ligand. In an EPA glass at liquid nitrogen temperature [RhX(P4)(PPh3)2] (X = Cl, Br) shows five absorptions in the 700-260-nm region. These are assigned to one-electron transitions, with good agreement between the observed and calculated energies. The absorptions owe most of their intensity to metal ? P4 and metal ? phosphine charge transfer. © 1986 American Chemical Society.

Original languageEnglish
Pages (from-to)403-416
Number of pages14
JournalJournal of the American Chemical Society
Issue number3
Publication statusPublished - 1986


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