Ab initio calculations suggest that the Sb and Bi atoms in group 15 form double bonded compounds, as do the lighter P and As atoms, unlike the heavier Sn and Pb in group 14; dibismuthenes are the heaviest example of double bonding between main group elements.Compounds featuring double bonding between the heavier group 15 elements are of current interest.1 In 1981, the first stable P-P double bonded compound, a diphosphene derivative (RP=PR), was synthesized and isolated.2 Since then, many schemes have been revised and diphosphenes are now commonplace. 1 In addition, compounds with an -As=Aslinkage, diarsenes, have also been successfully synthesized and is0lated.3-~ However, attempts to prepare the heavier analogues, distibenes (RSb=SbR) and dibismuthenes (RBi= BiR), have all been unsuccessful up to now, except for the metal-coordinated distibene complexes, but these lose double bond character because of side-on q2 coordination."9The lighter Si and Ge atoms in group 14 can form isolable ethene analogues ,lo disilenes (R2Si=SiR2)11 and digermenes (R2Ge=GeR2).I2 However, the heavier Sn is reluctant to form double bonds in distannenes ( R2Sn=SnR2) ,13,14 while Pb does not form a diplumbene (R2Pb=PbR2) structure at a11.15-1h From this point of view, it is important to determine if the heavier atoms in group 15 are also incapable of forming double bonded compounds. For this purpose, we have undertaken the first ab initio calculations of the parent compounds, HSb=SbH 3 and HBi=BiH 4.Geometries were fully optimized at the Hartree-Fock (HF) level with relativistic effective core potentials (ECP)17 on Sb and Bi using the double-zeta (DZ) basis sets17 augmented by a set of d-type polarization functions [d exponents18 0.211 (Sb) and 0.185 (Bi)]. The D Z basis set19 for H was scaled by a factor

By means of a6 initio calculations, unusually short interatomic distances are predicted between the nonbonded atoms in the title compounds and interpreted in terms of small extent of back electron-donation from the linked oxygens and the accompanying orbital contraction around the nonbonded atoms.

The MNDO MO calculations were carried out for the formation ofoxaphosphetanes from substituted ylides and aldehydes (Equations 2-4). It was found that the reactions proceed through a nearly planar cyclic fourmembered rit.zg transition state (TS) in all cases studied and that phenyl substitution on phosphorus has little effect on the TS geometry. This finding is not in line with the TS models that have been proposed to explain the experimentally observed cis-selectivity.