Metallacycles, especially those with high strain, have attracted interest because of their high reactivity. They occur as intermediates in synthesis or find application in catalysis.
Relieving the strain of four-membered rings is possible when metals or heteroatoms are included in the structure. This method was used to synthesize four-membered metallacycles containing a group 4 metal, nitrogen, and sulfur.

Eluvathingal Jemmis, Indian Institute of Science, Axel Schulz, Leibniz Institute for Catalysis and University of Rostock, Germany, Uwe Rosenthal, Leibniz Institute for Catalysis and University of Rostock, and co-workers prepared mononuclear unsaturated titanocene and zirconocene compounds with an N–S–N scaffold by reaction of titanocene and zirconocene alkyne complexes with sulfurdiimide Me₃SiN=S=NSiMe₃.

The titanocene compound was analyzed by Density Functional Theory (DFT) calculations and its structure was determined by X-ray crystallography, which revealed that the metallacycle was not planar but had a butterfly motif due to pseudo-Jahn–Teller distortion.

_{Images: © Wiley-VCH}

• Experimental and Theoretical Studies of Unusual Four-Membered Metallacycles from Reactions of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes with the Sulfurdiimide Me₃SiN=S=NSiMe₃,
  K. Kaleta, M. Ruhmann, O. Theilmann, S. Roy, T. Beweries, P. Arndt, A. Villinger, E. D. Jemmis, A. Schulz, U. Rosenthal,
  Eur. J. Inorg. Chem. 2012.
  DOI: 10.1002/ejic.201101188