Tertiary arsine adducts of iodoarsines: A structural and theoretical investigation

The Lewis acid-base adducts PhMe₂As→AsPhI₂, PhMe₂As→AsMeI₂, and PhMeEtAs→AsMeI₂ have been structurally characterized by single-crystal X-ray diffraction, and their structures and bonding investigated by density functional theory calculations at the PBE/TZP level of theory. The adduct PhMe₂- As→AsPhI₂ crystallizes in the monoclinic space group Pna2 ₁. The coordination geometry around the arsenic atom of the iodoarsine in the adduct is distorted trigonal bipyramidal with the arsenic atom of the tertiary arsine being almost directly above the arsenic and orthogonal to the T-shaped iodoarsine at a distance of 2.456(1) Å. The nearest intermolecular neighbor to the arsenic of the iodoarsine in the structure is the phenyl group of the tertiary arsine of an adjacent molecule. The adduct PhMe₂As→AsMeI₂ crystallizes in the monoclinic space group P2₁/c. The core structure of this adduct is a dimer based on edge-sharing through iodine atoms in the basal MeAsI₃ planes of two square pyramids in which the arsenic atoms of the tertiary arsines occupy the apical sites of the pyramids in a trans arrangement within the dimeric unit (As-As 2.4979(5) Å). The adduct PhMeEtAs→AsMeI₂ crystallizes in the space group Pbca. Intermolecular contacts between the arsenic of the T-shaped iodoarsine and an iodine of an adjacent molecule trans to the arsenic of the tertiary arsine generate helical -(-As-I-As-)-_n chains of opposite helicity running through the unit cell. The calculations indicate that the molecular orbitals most directly associated with the As-As bonding in the adducts do not show any significant contribution from the virtual orbitals of either arsenic subunit. The theoretical results, which give As-As bond energies of 10-30 kJ mol^-1 for the three adducts, are consistent with the ready dissociation of the adducts in solution and the importance of intermolecular interactions in stabilizing the complexes in the solid state.