The group 9 cyclopentadienylmetal cis-ethylenedithiolates as metallodithiolene ligands in metal carbonyl chemistry: analogies to benzene metal carbonyl complexes

Abstract

The experimentally known cobalt ethylenedithiolate complex CpCoS₂C₂H₂ as well as its rhodium and iridium analogues are aromatic systems that can function as pentahapto six-electron donor metallodithiolene ligands by binding through all five atoms of its CoS₂C₂ ring. The lowest energy CpMS₂C₂H₂·Cr(CO)_n (n = 3, 2) and CpMS₂C₂H₂·Fe(CO)₂ structures (M = Co, Rh, Ir) by substantial margins are predicted by density functional theory to have structures with such pentahapto η⁵-(CpMS₂C₂H₂) (M = Co, Rh, Ir) ligands. The CpMS₂C₂H₂·Cr(CO)₃ structures appear to be favorable as indicated by high carbonyl dissociation free energies (ΔG) exceeding 15 kcal mol⁻¹. The lowest energy structures for the carbonyl-richer CpMS₂C₂H₂·Cr(CO)₄ and CpMS₂C₂H₂·Fe(CO)₃ systems have trihapto CpMS₂C₂H₂ ligands involving a heterometallic metal–metal bond and two metal–sulfur bonds retaining an uncomplexed CC double bond in the ethylenedithiolate ligands. The CpMS₂C₂H₂·Fe(CO)₃ structures can be derived from the structures of the long-known (RS)₂Fe₂(CO)₆ derivatives by replacing one of the Fe(CO)₃ units by an isoelectronic/isolobal CpM unit. Such CpMS₂C₂H₂·Fe(CO)₃ structures appear to be favorable as indicated by high carbonyl dissociation free energies (ΔG) exceeding 15 kcal mol⁻¹. Higher energy CpMS₂C₂H₂·Cr(CO)₄ and CpMS₂C₂H₂·Fe(CO)₃ structures involve coordination of the CC double bond and at least one sulfur atom of the CpMS₂C₂H₂ ligand to the chromium or iron atom but have long metal–metal distances suggesting lack of a direct heterometallic metal–metal bond. In the lowest energy structures of the still carbonyl richer CpMS₂C₂H₂·Cr(CO)₅ and CpMS₂C₂H₂·Fe(CO)₄ the CpMS₂C₂H₂ ligand is a dihapto ligand that is bonded to the chromium or iron atom through a heterometallic M–M′ (M′ = Cr, Fe) bond and a metal–sulfur bond. Other types of dihapto ligand–metal bonding found in higher energy CpMS₂C₂H₂·Cr(CO)₅ and CpMS₂C₂H₂·Fe(CO)₄ structures include bonding only the CC double bond of the ethylenedithiolate ligand to the chromium or iron atom as in a simple (olefin)M(CO)_n complex or bonding of adjacent carbon and sulfur atoms in the CpMS₂C₂H₂ ligand to the chromium or iron atom. The latter mode of dihapto C,S bonding of the CpMS₂C₂H₂ ligand can be interpreted as a π-bond from a CS double bond in the dithioglyoxal structure for CpMS₂C₂H₂.