Dibismuthines structure

Syntheses and Solid State Structures of Group 13-Distibine and -Dibismuthine Bisadducts... 

The so-prepared compounds are stable in the pure form, whereas they easily undergo consecutive reactions in solution under cleavage of the E—E bond (see Sect. 3.1). Four tetraalkyldistibine (32-35) and two -dibismuthine bisadducts (36 and 37) have been structurally characterized (Table 8). 

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 sharp contrast to the intensely studied reactions of dipenteles with transition metal compounds, reactions with group 13 metal compounds are almost unknown. Only two diphosphine-borane bisadducts of Type C ([H3B]2[Me4P2], [H2(Br)B]2[Me4P2]) have been synthesized and structurally characterized65 but no diarsine, distibine or dibismuthine adducts. We, therefore, became interested in the synthesis of such compounds, focusing... 

Figure 5 Structures of GaBu 3 adducts of a distibine 104 and a dibismuthine 106.

Fewer structural data are available for dibismuthines. Published structures include only the thermochromic tetrakis(trimethylsilyl)dibismu-thine (37) (57) and the nonthermochromic tetraphenyldibismuthine (46) (42) although some data from an unpublished structure of tetramethyl-... 

While tetraphenyldibismuthine (46) is not isostructural with tetraphenyl-distibine (1), it does not show any close intramolecular Bi—Bi contacts. Although no other structural data are available, it might be noted that there is a complete match of the thermochromic properties for the dibis-muthines with the corresponding distibines. Thus, in all cases if a distibine is thermochromic the analogous dibismuthine is as well. Conversely, nonthermochromic distibines correspond to nonthermochromic dibis-muthines. A structural correspondence also seems likely. 

No theoretical treatment is available for dibismuthines. However, the qualitative features of the band structures of the thermochromic dibismuthines can be anticipated from the Hoffmann model for the corresponding distibines. Thus, the substitution of Bi for Sb is unlikely to affect greatly the ligand v orbitals. But the trg orbital will likely be raised to reflect the weaker metal-metal bond (28). A smaller band gap is predicted, as is observed. 

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. 

Table 2 summarizes important structural parameters for alane-stibine and -bismuthine adducts R3A1-ER 3, while Table 3 features those for distibines and dibismuthines precursors R2E-ER 2 (E = Sb, Bi) and the corresponding alane adducts. 

---