Final Exam

December 21, 2000

2. M. M. Hosain and K. Matsumoto (p. 247-250) used the pentasulfide anion, S₅^2–, as a tridentate ligand in the complex Ru(P(OCH₃)₃)₃S₅. Draw the Lewis dot structure of the pentasulfide anion (assume the octet rule is obeyed and there is a linear connectivity between sulfur atoms) and indicate the formal charge and formal oxidation number on each S atom. Name the metal complex. P(OCH₃)₃ is a neutral ligand named trimethylphosphite.

Answer:

This implies that pentasulfide is a dianion.

Pentasulfide most likely binds to a metal from the formally negative sites at the ends and, to behave as a tridentate ligand, one of the central S atoms (experimentally found to be the 2^nd S atom along the chain). This constrains the geometry of the metal complex so that only the fac isomer can form. Thus, the name of the complex is: fac-pentasulfide-tris-(trimethylphosphite)ruthenium(II)