The reactions of para-halo diaryl diselenides with halogens. A structural investigation of the CT compound (p-FC6H4)2Se2I2, and the first reported "RSeI3" compound, (p-ClC6H4)SeI·I2, which contains a covalent Se-I bond.

The reactions of the diaryl-diselenides (p-FC(6)H(4))(2)Se(2) and (p-ClC(6)H(4))(2)Se(2) with diiodine have been investigated. Species of stoichiometry "RSeI" are formed when the ratio employed is 1:1. The solid-state structure of "(p-FC(6)H(4))SeI" has been determined, and shown to be a charge-transfer (CT) adduct, (p-FC(6)H(4))(2)Se(2)I(2), where the Se-Se bond is retained and the diiodine molecule interacts with only one of the selenium atoms. The Se-I bond in (p-FC(6)H(4))(2)Se(2)I(2) is 2.9835(12) Å, which is typical for a (10-I-2) Se-I-I CT system. When diiodine is reacted in a 3:1 ratio with (p-XC(6)H(4))(2)Se(2) (X = F, Cl) species of stoichiometry "RSeI(3)" are formed. The structure of "(p-ClC(6)H(4))SeI(3)" reveals that this is not a selenium(IV) compound, but is better represented as a selenium(II) CT adduct, (p-ClC(6)H(4))SeI·I(2). The Se-I bond to the diiodine molecule is typical in magnitude for a CT adduct, Se-I: 2.8672(5) Å, whereas the other Se-I bond is much shorter, Se-I: 2.5590(6) Å, and is a genuine example of a rarely observed covalent Se-I bond, which appears to be stabilised by a weak Se···I interaction from a neighbouring iodine atom. The reaction of (p-ClC(6)H(4))SeI with Ph(3)P results in the formation of a CT adduct, Ph(3)PSe(p-ClC(6)H(4))I, which has a T-shaped geometry at selenium (10-Se-3). By contrast, the reaction of (p-FC(6)H(4))SeI with Ph(3)P does not form an adduct, but results in the formation of Ph(3)PI(2) and (p-FC(6)H(4))(2)Se(2).

Structural isomerism in (p-XC6H4)SeCl3 and (p-XC6H4)SeBr3 (X = F, Cl) compounds. Co-crystallisation of cis- and trans-dimeric forms of (p-ClC6H4)SeCl2(µ-Cl)2(p-ClC6H4)SeCl2. A new structural modification for the "PhSeBr" reagent, Ph2Se2Br2, containing an elongated Se-Se bond.

A series of di(para-halophenyl)diselenides, (p-XC(6)H(4))(2)Se(2) (X = F, Cl) have been reacted with three equivalents of SO(2)Cl(2) or Br(2), leading to the formation of selenium(iv) RSeX(3) compounds. The structures of (p-FC(6)H(4))SeX(3) (X = Cl, Br) have been determined, and both exhibit a dimeric RSeX(2)(µ-X)(2)RSeX(2) structure consisting of two "saw-horse" (p-FC(6)H(4))SeX(3) units linked by two halide bridges, with an overall square pyramidal geometry at selenium. In both structures all the selenium and halogen atoms are planar, with both aryl rings located on the same side of the Se(2)X(6) plane (cis-isomer). The structure of (p-ClC(6)H(4))SeCl(3) also adopts a planar dimeric structure, however both cis- and trans-dimeric molecules are co-crystallised in the unit cell. In contrast, the structure of (p-ClC(6)H(4))SeBr(3) adopts a folded cis-dimeric structure due to steric constraints. Secondary Se···X interactions to the "vacant" sixth coordination site at selenium are a feature of most of these structures, but are most prominent in the folded structure of (p-ClC(6)H(4))SeBr(3). A re-examination of the PhSeBr/PhSeBr(3) system resulted in the isolation of crystals of a second structural form of "PhSeBr". The structure of Ph(2)Se(2)Br(2) consists of two PhSeBr units linked by an elongated Se-Se bond of 2.832(4) Å, and longer secondary Se···Br interactions of 3.333(4) Å to form a chain structure. Further weak Se···Br and Br···Br interactions are present, which form loosely linked rippled sheets of selenium and bromine atoms, similar to the sheets observed for the tetrameric form, Ph(4)Se(4)Br(4).

Structural isomerism in tris-tolyl halo-phosphonium and halo-arsonium tri-halides, [(CH3C6H4)3EX][X3], (E = P, As; X = Br, I).

The group 15 ligands (o-CH(3)C(6)H(4))(3)P, (m-CH(3)C(6)H(4))(3)P, (p-CH(3)C(6)H(4))(3)P, Ph(3)As, (o-CH(3)C(6)H(4))(3)As and (p-CH(3)C(6)H(4))(3)As have been reacted with two equivalents of di-iodine or di-bromine to yield complexes of formula R(3)EX(4) (E = P, As; X = I, Br). These halogenated group 15 compounds are ionic, [R(3)EX][X(3)] consisting of halo-phosphonium or halo-arsonium cations and trihalide anions. These adducts exhibit structural isomerism and may exist either as simple 1:1 ion pairs, [R(3)EX][X(3)], isomer (A), which display a weak XX interaction between cation and anion, or as a 2:1 complex, which consists of a [{R(3)EX}(2)X(3)](+) cationic species made up of two [R(3)EX](+) cations interacting with one [X(3)](-) anion. The overall charge is balanced by a second [X(3)](-) anion. These 2:1 species also exhibit structural isomerism due to subtle differences in the connectivity of the [{R(3)EX}(2)X(3)](+) fragment, as the {R(3)EX}(+) units may either interact at the same end of the [X(3)](-) ion, to give a Y-shaped motif, isomer (B), or at opposite ends, giving a Z-shaped motif, isomer (C). The type of structural isomer formed is related to the way in which [Ar(3)EX](+) cations pack together via aryl embraces. Isomer (A) and (C) structures form chains of side-to-side, anti-parallel embracing cations. In (A) and (C) structures a square-like stacking motif of cations is observed. In contrast, isomer (B) structures feature side-to-side, parallel embracing cations, and do not exhibit the square motif.