A study of the halogen...halogen contacts in organic compounds using ab initio calculations and the results of previously reported crystallographic studies show that these interactions are controlled by electrostatics. These contacts can be represented by the geometric parameters of the C--X1...X2--C moieties (where theta1=C--X1...X2 and theta2=X1...X2--C; ri=X1...X2 distance). The distributions of the contacts within the sum of van der Waals radii (rvdW) versus thetai (theta1=theta2) show a maximum at theta approximately 150 degrees for X=Cl, Br, and I. This maximum is not seen in the distribution of F...F contacts. These results are in good agreement with our ab initio calculations. The theoretical results show that the position of the maximum depends on three factors: 1) The type of halogen atom, 2) the hybridization of the ipso carbon atom, and 3) the nature of the other atoms that are bonded to the ipso carbon atom apart from the halogen atom. Calculations show that the strength of these contacts decreases in the following order: I...I>Br...Br>Cl...Cl. Their relative strengths decrease as a function of the hybridization of the ipso carbon atom in the following order: sp2>sp>sp3. Attaching an electronegative atom to the carbon atom strengthens the halogen...halogen contacts. An electrostatic model is proposed based on two assumptions: 1) The presence of a positive electrostatic end cap on the halogen atom (except for fluorine) and 2) the electronic charge is anisotropically distributed around the halogen atom.

The synthesis and characterization of several m-terphenyl heavier main group 15 (P, As, Sb, or Bi) dihalides, together with their reduction to give a homologous series of double-bonded dipnictenes, are reported. Reaction of LiC6H3-2,6-Mes2 (Mes = C6H2-2,4,6-Me3) or LiC6H3-2,6-Trip2 (Trip = C6H2-2,4,6-iPr3) with the appropriate trihalide affords 2,6-Mes2H3C6ECl2 (E = As, 1; Sb, 2; Bi, 3) and 2,6-Trip2H3C6ECl2 (E = P, 4; As, 5; Sb, 6; Bi, 7). The compounds 1 − 7 were characterized by 1H and 13C NMR spectroscopy as well as by 31P NMR spectroscopy in the case of 4. In addition, the structures of 3, 5, and 6 were determined. Reduction of the phosphorus species 4 with potassium in hexane gives a mixture of the diphosphene 2,6-Trip2H3C6PPC6H3-2,6-Trip2, 12, and the phosphafluorene species, 1-(2,4,6-triisopropylphenyl)-5,7-diisopropyl-9-phosphafluorene, 11. The compound 11, which results from the insertion of a phosphorus into a C(Ar)−C(i-Pr) bond was synthesized in higher yield by the reduction of 4 with magnesium. The simple reduction of 1 − 4, 6, and 7 with potassium, and of 5 with magnesium, yielded the new series of dipnictenes, 2,6-Mes2H3C6E=EC6H3-2,6-Mes2 (E = As, 8; Sb, 9; Bi, 10) and 2,6-Trip2H3C6E=EC6H3-2,6-Trip2 (E = P, 12; As, 13; Sb, 14a; Bi, 15), as well as the partially reduced species 2,6-Trip2H3C6(Cl)SbSb(Cl)C6H3-2,6-Trip2 (14b). The compounds, which displayed high thermal stability, were characterized by 1H, 13C, and 31P NMR and UV−vis spectroscopy. The structures of 8 − 11, 13, 14a, and 14b were determined. These compounds constitute the first homologous series of dipnictene structures for all the heavier group 15 elements. The E−E bond shortenings observed for the heaviest antimony or bismuth derivatives lead to the conclusion that π overlap is quite important in the fifth- and sixth-period elements of this group.

The mixed-ligand complex [Ni(dppp)(P(Ph)(2)N(Bz)(2))](BF(4))(2), 3, (where P(Ph)(2)N(Bz)(2) is 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane and dppp is 1,3-bis(diphenylphosphino)propane) has been synthesized. Treatment of this complex with H(2) and triethylamine results in the formation of the Ni(0) complex, Ni(dppp)(P(Ph)(2)N(Bz)(2)), 4, whose structure has been determined by a single-crystal X-ray diffraction study. Heterolytic cleavage of H(2) by 3 at room temperature forms [HNi(dppp)(P(Ph)(2)N(Bz)(mu-H)N(Bz))](BF(4))(2), 5a, in which one proton interacts with two nitrogen atoms of the cyclic diphosphine ligand and a hydride ligand is bound to nickel. Two intermediates are observed for this reaction using low-temperature NMR spectroscopy. One species is a dihydride, [(H)(2)Ni(dppp)(P(Ph)(2)N(Bz)(2))](BF(4))(2), 5b, and the other is [Ni(dppp)(P(Ph)(2)N(Bz)(2)H(2))](BF(4))(2), 5c, in which both protons are bound to the N atoms in an endo geometry with respect to nickel. These two species interconvert via a rapid and reversible intramolecular proton exchange between nickel and the nitrogen atoms of the diphosphine ligand. Complex 3 is a catalyst for the electrochemical oxidation of H(2) in the presence of base, and new insights into the mechanism derived from low-temperature NMR and thermodynamic studies are presented. A comparison of the rate and thermodynamics of H(2) addition for this complex to related catalysts studied previously indicates that for Ni(II) complexes containing two diphosphine ligands, the activation of H(2) is favored by the presence of two positioned pendant bases.