PENTACOORDINATE ZINC - SYNTHESIS AND STRUCTURES OF BIS[1-(METHYLTHIO)-CIS-STILBENE-2-THIOLATO]ZINC AND OF ITS ADDUCTS WITH MONODENTATE AND BIDENTATE NITROGEN BASES

1-(Methylthio)-cis-stilbene-2-thiol, Ph(SCH3)C = C(SH)Ph, forms a dimeric mu-S-bridged zinc complex of composition Zn2-[(CH3)S2C2Ph2]4, containing equivalent pentacoordinate Zn(II) ions in a trigonal-bipyramidal ligand environment; Zn-S bond lengths vary from 2.270 (3) to 2.712 (3) angstrom (2.46 angstrom average), as determined by single-crystal X-ray analysis. It reacts with 4-(dimethylamino)pyridine to yield a monomeric adduct in which Zn(II) is also pentacoordinate and the coordination geometry is distorted square-pyramidal with Zn-N and average Zn-S bond lengths of 2.069 (10) and 2.48 (25) angstrom. In the 2,2'-bipyridine (bpy) adduct, the zinc atom is pentacoordinate as well, although one coordinate Zn-S(CH3) bond is weak. The bpy ligand is symmetrically coordinated to zinc, with mean Zn-N bond distances of 2.097 (10) angstrom. One sulfur ligand is asymmetrically bidentate with one normal Zn-S bond of 2.292 (3) angstrom; the coordinate Zn-S(CH3) bond of 3.124 (3) angstrom is long but still shorter than the sum of the van der Waals radii. The other sulfur ligand is monodentate, the Zn-S bond length is 2.275 (3) angstrom, and the -S-CH3 group is in a noninteractive position. The nitrogen ligands exchange with free ligands in solution, as evidenced by variable-temperature H-1 NMR measurements. The preferred adoption by Zn(II) of pentacoordinated structures under conditions where tetra- and hexacoordination are possible with identical probabilities suggests that pentacoordination is energetically favored and may be important in biological systems.