Distibines, conformation

Structures 5 and 6 display the solid state structures of two representative distibine and dibismuthine adducts. The ligands bound to the central Sb and Bi atoms adopt a staggered conformation in relation to one another, with the bulky M(t-Bu)3 groups arranged in a trans-position. This is likely due to repulsive steric interactions. The central Sb—Sb [281.1(1) 32 283.9(1) pm 35] and Bi—Bi bond distances [298.3(1) 36 and 298.4(1) pm 37] are almost unchanged compared to the uncomplexed distibines and dibismuthines, as can be seen... 

In order to describe the different conformations in distibines or in distibine complexes, the torsion angles Ep-Sb-Sb-Ep (cp) for the free ligand... 

The bis-adducts are stable in the solid state at low temperatures. They dissociate in solution at ambient temperature. In the case of Me2Sb-SbMe2 a stable complex was formed only with the sterically hindered and less electrophilic t-Bu3Al. X-ray crystal structure analyses for [( -Bu)3M(R2Sb-SbR2)M( -Bu)3] (R = Me, Et M = Al, Ga) revealed that the distibine ligands adopt the anti conformation (Structural parameters of distibine complexes are summarized in Table I. 

Many distibines and dibismuthines have lighter colors in solutions or melts than in the solid state. Crystals of these thermochromic distibines or dibismuthines consist of linear chains of the dimetal compounds with short intermolecular metal-metal contacts. Delocalization of electrons along the chains is possibly responsible for the bathochromic shift between fluid and solid phases. Usually, the /raor-conformation is adopted by the tetraorganodimetal compounds in the solid state. (CF3)4As2 shows the /ra r-conformation also in the gas phase. Photoelectrospectroscopic measurements on Me4Sb2 revealed the presence of gauche- (12%) and trans- (88%) conformed in the gas phase.52... 

The five distibines (1, 22, 30, 36, and 40) that have been investigated by X-ray crystallography show very similar molecular structures (see Fig. 2 for the molecular structure of 2,2, 5,5 -tetramethylbistibole, 30). In each case the distibine adopts a staggered trans conformation so that there is an inversion center through the Sb—Sb bond. The Sb—Sb bond lengths vary between 2.84 and 2.88 A, values somewhat shorter than that of elemental antimony (2.90 A) (43). The bond angles about antimony are close to 90° and similar to those reported for other trivalent antimony compounds (44). In fact, none of the molecular parameters seems extraordinary. However, the four thermochromic distibines show unusual crystal packing (see Fig. 3, Table V) (22,25,36,40,41,45). In each case, the antimony atoms are... 

No diarsines or diphosphines show thermochromic behavior similar to that of the distibines and dibismuthines. For example, in the series of dipnictogen compounds A, B, and C illustrated in Scheme 6, the thermochromic distibines and dibismuthines correspond to nonthermochromic diarsines and diphosphines. Structural data are available to compare the three diarsines 51 (46), 52 (57), and 53 (5S) with the corresponding distibines 22 (25,26), 36 (37), and 30 (22). Diarsines 52 and 53 crystallize in gauche conformations as opposed to the trans-staggered conformation of the distibines. In neither case are there intermolecular As---As contacts shorter than 4 A. However, the lack of conformational correspondence makes any comparison tenuous. 

A number of simple diarsines have been structurally investigated and, although individual molecules are similar to those of the corresponding distibines, the diarsines do not show the extended chain structures and consequent thermochromism often found for the latter. The geometry at the two arsenic atoms is pyramidal, though the angles at arsenic can be asymmetric. The substituents occupy anti (gauche) conformations and in some cases the molecules have imposed Cj symmetry. Parameters for four of the compounds are summarized in Table 2. 

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