Another example of the conformation-determining attractive interaction between gold@) centers is presented, based on changes in the conformation of the butadiene skeleton. The s-trans ground state geometry of 2,3-bis(diphenylpiiosphino)-l,3-butadiene (2) is abandoned in the gold complex 3, and the two gold atoms axe brought into contact a t a distance of 3.023(1)A. The conformational change of the ligand also occurs at a single metal atom, where the bisphosphine acts as a difunctional chelating ligand, as demonstrated in the rhodium complex 4.Structural 'v2' and spectroscopic ~t u d i e s~.~' on mono-and polynuclear gold(1) compounds have provided indirect evidence of an attractive interaction between the gold atoms. This weak bonding appears to be operative perpendicular to the linear X -Au -L axes and leads either to a catenated or layered aggregation of the mol-:cules by Au...Au contacts, which are ca. 3.0 A in length, or to the intramolecular pairing of the gold atoms with similar equilibrium distances. It can be described as a 5d"-5d" interaction based on a mixing with the 6s' states5', whose energy difference is lowered by relativistic effects6.''. In the case of the neighbour element mercury these effects are less obvious and restricted to only a few examples*'. However, recent work has shown that Au .. . Hg interactions can also be detected in complexes tailored after the Au ... Au precedents 9! In our search for further experimental evidence and for means to determine directly or indirectly the strength of this force quantitatively, initially doubly phosphino-substituted ylides 1 where chosen as model system. These compounds change their ground state synlanti orientation to a symmetrical synlsyn conformation upon double complexation with AuC12". In the complex the two gold (1) xntres are seperated by 3.000(1) A. From temperature-dependent NMR studies the energy of the Au...Au interaction can be estimated to be in the order of 7-8 kcal/mol.Au".-.\u 1 CI ci lo ' We report in this paper another example of the conformationietermining attractive interaction between gold(1) centers, based on :hanges in the conformation of the butadiene skeleton. The ground ;tate geometry of butadiene is determined by the conjugation of :he two x systems, and only two conformers represent energy minma in the energy profile of C2-C3 rotation.*I X-ray crystal structure analysis. 893At room temperature the s-trans form is clearly favoured over the s-cis form (s-cisls-trans ratio 1 :99")). As a consequence of the preferred s-trans conformation butadiene shows low reactivity in Diels-Alder [4 + 21 cycloadditions, which require the s-cis arrangement. According to electron diffraction ''), microwave ' ' I, NMR 13), UV l4l, infrared and Raman Is) data, and the results of force-field and a b initio calculations 16) the s-trans form of the unsubstituted butadiene is about 3 kcal/mol more stable than the s-cis isomer, and rotation requires an activation energy of 6-8 kcal/mol. The s-cis conformation shows a gauche arrangement with a to...

The title compound 4 has been prepared from readily available 2,3‐bis(diphenylphosphinyl)‐1,3‐butadiene (1) through double cyclopropanation using Me2S(CH2)O to give 1,1′‐bis(diphenylphosphinyl)bicyclopropyl (2), followed by reduction using HSiCl3/NEt3. Addition of sulfur to compound 4 yields the disulfide 5, and reaction with tetrahydrofuran – borane affords the 1:2 adduct with BH3 (6). Quaternization reactions with MeI or CH2I2 give the double quaternary salts 7 and 8, respectively. Single dehydrohalogenation employing nBuLi converts 8 into the cyclic semiylide salt 9. 4 is an excellent ligand for lowvalent late transition‐metal cations. With PdI2 the 1:1 complex LPdI2 (10, with L = 4), and with [(CO)2RhCl]2 the ionic 2:1 complex L2Rh+Cl‐ (11) are obtained. Experiments with (CO)AuCl yield the 1:2 complex L(AuCl)2 (12), X‐ray structure analyses were performed with single crystals of the disulfide 5, as well as the rhodium(I) and gold(I) complexes 11 and 12. 5 has a conformation between s‐cis and s‐trans with the PS functions pointing away from each other at opposite ends of the molecule. By contrast, in the gold(I) complex the ligand approaches an s‐cis conformation, and through rotations about P–C and C–C bonds – as referred to the conformation of 5 – the metal atoms are brought into close contact: Au…Au = 3.085 Å. Through temperature‐dependent NMR investigations of compounds 5 and 12, and by comparison with values calculated or experimentally determined for related bicyclopropyl compounds (available in the literature), the energy of the Au…Au attraction has been estimated to be ca. 6 kcal/mol. Compound 11 features a square‐planar, double‐chelate cation.

The title compound (VI) is synthesized from the diene (I) via the intermediates (IIb) (X‐ray analysis: space group P21/c; Z=2) and (III) as shown.

ChemInform Abstract The s-trans ground state geometry of the butadiene derivative (I) is abandoned in the gold complex (III). In order to determine the solid-state configuration of the butadiene skeleton, an X-ray analysis of (III) (P21, Z=2) is carried out. The conformational change of the ligand also occurs at a single metal atom, where the biphosphine acts as a difunctional chelating ligand, as demonstrated in the Rh complex (V).

The title compound (IV) reacts with S8 to give the disulfide (V) (space group P21, Z=2) and with THF·BH3 to yield a 1:2 adduct with BH3.