Two series of sterically crowded peri-substituted acenaphthenes have been prepared, containing mixed halogen-chalcogen functionalities at the 5,6-positions in A1-A6 (Acenap[X][EPh] (Acenap = acenaphthene-5,6-diyl; X = Br, I; E = S, Se, Te) and chalcogen-chalcogen moieties in A7-A12 (Acenap[EPh][E'Ph] (Acenap = acenaphthene-5,6-diyl; E/E' = S, Se, Te). The related dihalide compounds A13-A16 Acenap[XX'] (XX' = BrBr, II, IBr, ClCl) have also been prepared. Distortion of the acenaphthene framework away from the ideal was studied as a function of the steric bulk of the interacting halogen and chalcogen atoms occupying the peri-positions. The acenaphthene series experiences a general increase in peri-separation for molecules accommodating heavier congeners and maps the trends observed previously for the analogous naphthalene compounds N1-N12 (Nap[X][EPh], Nap[EPh][E'Ph] (X = Br, I; E/E' = S, Se, Te). The conformation of the aromatic ring systems and subsequent location of p-type lone-pairs dominates the geometry of the peri-region. The differences in peri-separations observed for compounds adopting differing conformations of the peri-substituted phenyl group can be correlated to the ability of the frontier orbitals of the halogen or chalcogen atoms to take part in attractive or repulsive interactions. Density-functional studies have confirmed these interactions and suggested the onset of formation of three-centre, four-electron bonding under appropriate geometric conditions.

Four novel 1,8-disubstituted naphthalene derivatives 4-7 that contain chalcogen atoms occupying the peri positions have been prepared and fully characterised by using X-ray crystallography, multinuclear NMR spectroscopy, IR spectroscopy and MS. Molecular distortion due to noncovalent substituent interactions was studied as a function of the bulk of the interacting chalcogen atoms and the size and nature of the alkyl group attached to them. X-ray data for 4-7 was compared to the series of known 1,8-bis(phenylchalcogeno)naphthalenes 1-3, which were themselves prepared from novel synthetic routes. A general increase in the E...E' distance was observed for molecules containing bulkier atoms at the peri positions. The decreased S...S distance from phenyl-1 and ethyl-4 analogues is ascribed to a weaker chalcogen lone pair-lone pair repulsion acting in the ethyl analogue due to the presence of two equatorial S(naphthyl) ring conformations. Two novel peri-substituted naphthalene sulfoxides of 1, Nap(O=SPh)(SPh) 8 and Nap(O=SPh)(2) 9, which contain different valence states of sulfur, were prepared and fully characterised by using X-ray crystallography and multinuclear NMR spectroscopy, IR spectroscopy and MS. Molecular structures were analysed by using naphthalene ring torsions, peri-atom displacement, splay angle magnitude, S...S interactions, aromatic ring orientations and quasi-linear O=S...S arrangements. The axial S(naphthyl) rings in 8 and 9 are unfavourable for S...S contacts due to stronger chalcogen lone pair-lone pair repulsion. Although quasi-linear O=S...S alignments suggest attractive interaction is conceivable, analysis of the B3LYP wavefunctions affords no evidence for direct bonding interactions between the S atoms.

The compound NH(SePPh,), has been synthesised from NH(PPh,), and selenium in refluxing toluene. A range of complexes [M{N(XPPh,),-X, X'},] (X = Se, M = Pt 1 or Pd 2; X = S, M = Pt 3, Pd 4 or Ni 5) of the monoanions derived from NH(SePPh,), and its sulfur analogue have been prepared. The new compounds have been characterised by microanalysis, NMR and IR spectroscopy. The crystal structures of NH(SePPh,),, 1, 2 and 5 were determined. The neutral NH(SePPh,), crystallised as H-bonded dimer pairs with a noticeable difference in the hydrogen bonded and nonhydrogen bonded P=Se bond lengths. The P=Se groups are approximately anti. On complexation the anionic ligands are bidentate and in all cases the resulting MX,P,N rings are distinctly puckered.

A new and improved method for the preparation of (PhPSe2)2 (Woollins reagent (WR), 1) is reported. Reaction of dichlorophenylphosphine with Na2Se, (prepared from the reaction of elemental selenium and sodium in liquid ammonia) gives WR with excellent purity, high yield and on a larger scale than was previously possible. Four novel phosphorus-selenium heterocycles, including a spirocyclic heterocycle exhibiting a four-membered P2SeC ring, were obtained from the reaction of WR with two reactive substrates (diphenylcyclopropenone and methyl phenylpropiolate). Useful selenocarbonyl and thiocarbonyl compounds were obtained from the reaction of both WR and Lawesson's reagent with diphenylcyclopropenone. All new compounds were characterised spectroscopically and three demonstrative X-ray structures are reported.

Synthetic and bonding aspects of heavier Group 15 (P, As, Sb, Bi) and 16 (S, Se, Te) peri-substituted naphthalenes, are discussed in this review. An important and unifying feature of the chemistry of these systems is the lively discussion about the nature of the interaction between peri-atoms. Are atoms bonded when they are closer than the sum of their van der Waals radii? Is there any (weak) bonding, or just a strained repulsive interaction? Positioning atoms of Group 15 and 16 at the naphthalene 1,8-positions provides leading systems with which to study these bonding issues.