Three-centre, four-electron bonding and structural characteristics of two-coordinate iodine(I) complexes with halogen and chalcogen ligands. Synthesis, spectroscopic characterization and x-ray structural studies of (triiodo)[tris(dimethylamino)phosphaneselenide]iodine and bis{(triiodo)[tri(N-morpholyl)phosphaneselenide]iodine(I)}/diiodine molecular complex

The two title compounds have been synthesized and their structures have been studied by X-ray crystallographic methods. The crystals of (triiodo)[tris (dimethylamino) phosphaneselenide] iodine(I) (I) are orthorhombic; at 103 K, a=14.003(4), b=10.958(2), c=11.801(2)Angstrom, Z=4, space group Pna2(1), final conventional R=0.023. The crystals of bis{(triiodo)[tri(N- morpholyl)phosphaneselenide]iodine (I)}/diiodine (II) are monoclinic; at 103 K a=8.999(4), b=16.874(8), c=16.996(6) Angstrom, beta=103.98(3)degrees, Z=4, space group P2(1)/c, final conventional R=0.033. The two compounds represent hypervalent complexes of I-I with nearly linear Se-I-I-I sequences with Se-I bond lengths of 2.596(1) and 2.590(1) Angstrom for I and II, respectively. Corresponding I-I-I bonds are weakened and their bond lengths are 3.215(1) and 3.186(1)Angstrom. The bonding of I and II is studied within the framework of three-centre, four-electron bonding (3c,4e) centered on the I-I atom. Based on a literature survey, we have found many similar compounds mostly with the sequence Y-I-I-X, where Y=S or Se and X = halogen. The majority of these compounds fit well into this 3c,4e scheme. The relative trans-influences of the chalcogen ligands are found to be dependent upon the ligand's ability to accept positive charge from the central I-I atom. A systematic study of the inter-relationship between bond lengths Y-I and I-X shows that this depends on secondary interactions of Y and X in a predictable manner.