Stereochemical role of lone pairs in main-group elements. Part 2. Structure and bonding in trichloro(tetramethylthiourea)antimony(III) studied by means of X-ray crystallography and antimony-121 Mössbauer spectroscopy

Abstract

The crystal structure of the title compound has been determined from X-ray diffractometer data by Patterson and Fourier methods. Crystals are monoclinic, space group Cc, with Z= 4 in a unit cell of dimensions a= 10.180(2), b= 15.306(2), c= 7.699(1)Å, and β= 105.48(1)°. The structure was refined to R 0.019 for 1 426 observed reflections. The antimony(III) atoms are in distorted trigonal-bipyramidal sites surrounded by three chlorine atoms at 2.428(2), 2.527(3), and 2.687(2)Å and by a tetramethylthiourea sulphur atom at 2.505(2)Å. The stereochemistry and distortion at the antimony(III) site is consistent with the lone pair occupying an equatorial position. The ¹²¹Sb Mössbauer parameters are reported and compared with literature values for related complexes. The increased negative shifts on going from SbCl₃ to the complexes, together with the signs and magnitudes of the quadrupole interactions, are interpreted within a covalent framework in terms of the use of antimony 5s and 5p valence orbitals and the desire of these systems to lower their total energies by distortions which maximise the populations of the lower-energy 5s orbital.