Comparison of stereochemistry in racemic and optically resolved forms of a five-co-ordinate compound. Crystal and molecular structures of dibromotris-(2-phenylisophosphindoline)palladium(II)–acetone (orange) and dibromotris-(2-phenylisophosphindoline)palladium(II)(red)

Abstract

The form of the molecule PdL₃Br₂(L = 2-phenylisophosphindoline) has been determined by crystal structure analysis of the optically resolved red form (B), and of an orange racemate (A), a 1 : 1 solvate with acetone. In both structures the molecule has a modified tetragonal pyramidal arrangement of palladium bonds with one bromine atom at the apex. In both the angle α, between the apical bond and the basal bond, is considerably greater than 90° for the basal bromine and the phosphorus atom trans to it. To one of the other phosphorus atoms α is noticeably less than 90° and to the remaining phosphorus atom it is slightly greater than 90°. The form is derived from the tetragonal pyramidal where all α are several degrees greater than 90° by a decrease in α for P(I) and P(III) while α for P(II) and Br(1) is free to increase. This distortion arises from overcrowding of the basal atoms.

In nearly all respects the molecule (B) differs insignificantly from the same enantiomorph in (A), but the apical bond length to bromine is 2·92 in (B) and 3·02 Å in (A). The large difference can only be attributed to difference of molecular environment. The angle α for the basal bromine is smaller for (A) and this may be a consequence of the bond-lengthening.

Crystals of (A) are monoclinic, space group P2₁/b, a= 11·36(2), b= 22·52(4), c= 16·67(3)Å, γ= 95·51 (2)°, Z= 4. Crystals of (B) are orthorhombic, space group P2₁2₁2₁, a= 11·63(2), b= 16·36(3), c= 20·48(3)Å, Z= 4. The structures were solved from diffractometer data [(A), Patterson and Fourier, 4989 observed reflections; (B), direct methods, 2822 observed reflections] and refined by least squares to R 0·064 (A) and 0·065 (B).