Sulfur ligands thiocarbonylation

Figure 1.12 suggests that for carbonyl complexes the HOMO is localized primarily on the metal centre, with only a modest contribution from oxygen orbitals. Thus by far the majority of reactions of metal carbonyls with electrophiles involve direct attack at the metal, with the carbonyl serving as a spectator ligand. If, however, the metal centre is (i) particularly electron rich and (ii) sterically shielded and the electrophile is hard (in the HSAB sense) and also sterically encumbered, then attack may occur at the oxygen. Thiocarbonyls (LM-CS) are stronger 71-acids than CO and the sulfur is both softer and more nucleophilic. Thus electrophilic attack at the sulfur of thiocarbonyls is more common if the metal centre is electron rich (vcs < 1200 cm-1). Similarly, coordinated isocyanides (CNR) are more prone to attack by electrophiles at nitrogen. This is noteworthy in the sense that free isocyanides are attacked by electrophiles at carbon (Figure 3.19). The resulting carbyne ligands will be discussed in Chapter 5.

Carbon monoxide binds weakly as a cr-donor ligand to four-coordinate co-balt(II) systems. Despite a bout of artifactual excitement, CO has never been observed to bind significantly to five-coordinate Co systems with a nitrogenous axial base to yield octahedral six-coordinate species. - The sulfur analogue thiocarbonyl (CS), although not stable as a free entity, binds very strongly to iron-porphyrin species in a linear manner. ... 

An example of thione coordination is demonstrated with the thiocarbonyl donor ligand 2,6-dimethyl-4H-pyran-4-thione, that gives a complex with zinc chloride of the form ZnL2Cl2-The crystal structure shows the zinc is tetrahedrally coordinated by two chloride and two sulfur donors and the compound was further studied by 3H NMR and IR spectroscopy.573 Both monomeric and dimeric heterocyclic thione complexes have been formed with zinc.5 4,575... 

Several diatomic ligands similar to CO are worth brief mention. Two of these, CS (thio-carbonyl) and CSe (selenocarbonyl), are of interest in part for purposes of comparison with CO. In most cases, synthesis of CS and CSe complexes is somewhat more difficult than for analogous CO complexes, because CS and CSe do not exist as stable, free molecules and do not, therefore, provide a ready ligand source.Therefore, the comparatively small number of such complexes should not be viewed as an indication of their stability. Thiocarbonyl complexes are also of interest as possible intermediates in certain sulfur transfer reactions in the removal of sulfur from natural fuels. In recent years, the chemistry of complexes containing these ligands has developed more rapidly as avenues for their synthesis have been devised. 

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