Preparation, ligand-exchange reactions, and alkylation reactions of some carbon disulphide derivatives of iron

Abstract

A number of [Fe(CO)₂L₂(η²-CS₂)] complexes have been prepared from [Fe₂(CO)₉] and L in CS₂ solution [L = PR₃, P(OR)₃, or CNR]. When L = P(OMe)₃ a second unstable product is isolated which may be polynuclear. When L = CNMe or CNBu^t further reaction occurs to give [Fe(CO)L₃(CS₂)] and then a carbonyl-free complex. Ligand-exchange reactions of the complexes where L = PPh₃ or P(OPh)₃ with the phosphorus(III) ligands L′ gives [Fe(CO)₂L(L′)(CS₂)] rapidly and then [Fe(CO)₂L′₂(CS₂)] much more slowly. The extent of this reaction depends on ligand size and is more complete for the less bulky L′. However, ligand basicity is also very important so that L′= AsBuⁿ₃ displaces only one L = P(OPh)₃ whilst with L′= SbEt₃ even this reaction is incomplete. On the other hand the very bulky but basic L′= PPrⁱ₃ will displace completely the less bulky but less basic L = P(OPh)₃. Where L′= CNMe the reaction takes a different course with the formation of [Fe(CO)₂L(CNMe)(CS₂)] and then [Fe(CO)L(CNMe)₂(CS₂)]. Reagents such as L′= CO, SO₂, [N₂Ph]⁺, and X₂(halogen) will displace CS₂ to give [Fe(CO)₂L₂(L′)]. The η²-CS₂ complexes are alkylated readily at the unco-ordinated sulphur atom, S_u, forming [Fe(CO)₂L₂(CS₂R)]⁺ salts which are relatively inert to exchange of L. Carbon disulphide is a powerful electron-withdrawing ligand. It is suggested that this is because it may accept electrons into (a) its C–S_Cπ* orbital as in olefin–metal bonding, (b) the vacant 3d orbitals on the co-ordinated sulphur atom S_c as in thioether–metal bonding, and (c) the C–S_uπ* orbitals as in M–CS bonding with this last being particularly important. Also, it is suggested that, as a consequence of the importance of the metal-to-CS₂ backbonding, η²-CS₂ complexes are only stable if the metal centre is relatively electron-rich. Thus phosphorus ligands which are very basic but bulky, e.g. PPrⁱ₃, will replace those which are less basic even if they are less bulky, e.g. P(OPh)₃.