Transition metal complexes of sulfur monoxide

Transition metal complexes of sulfur monoxide refers to coordination complexes with sulfur monoxide (SO) as a ligand. The topic is relevant to the metal-promoted redox reactions of sulfur and sulfur oxides. Sulfur monoxide is unstable in condensed form, so its complexes are almost always prepared indirectly, e.g., using reagents that release SO.

Bonding modes

Structure of [Fe₃(SO)(CO)₉]^2-.^[1] Color code: red = O, yellow = S.

SO can bond to metals in a number ways:^[2][3]

• a terminal ligand, with a bent M−S−O arrangement, analogous structurally to "bent" nitrosyl. In such cases, SO is a Lewis base. An example is IrCl(P(i-Pr)₃)₂(SO) (i-Pr = isopropyl).^[4]
• triply bridging as in Fe₃(μ³-S)(μ³-SO)(CO)₉. In such cases SO is a 4-electron donor.
• Bridging with the μ-η¹,η¹-SO motif.^[5] Another bridging mode is μ-η¹,η²-SO, as observed in some Rh₂ complexes.^[6]

Complexes with the M−O−S arrangement have been observed using the techniques of laser ablation and low temperature spectroscopy.^[7][8]

Synthesis

SO complexes can be prepared from ethylene episulfoxides. This heterocycle binds metals through S, then releases ethylene.^[2] Lewis acid adducts of sulfinylamines release SO when heated. In this way [RhCl(SO)(PPh₃)₂]₂ can be produced from RhCl(PPh₃)₃ (Ph = C₆H₅).^[9]

Another approach involves redox reactions of metal sulfides. Some cases involve well-defined oxidants such as peroxides. In other cases, air is assumed to be the oxidant.^[5]