Dihalocarbene complexes structure

Dihalocarbene complexes are useful precursors to new carbenes by nucleophilic displacement of the chlorine substituents. This has been nicely illustrated for Fe(TPP)(=CCl2) by its reaction with two equivalents of Re(CO)5J to give the unusual /t-carbido complex Fe(TPP)=C=Re(CO)4Re(CO)5 which also contains a rhenium-rhenium bond. " The carbido carbon resonance was observed at 211.7 ppm in the C NMR spectrum. An X-ray crystal structure showed a very short Fe=C bond (1.605(13) A, shorter than comparable carbyne complexes) and a relatively long Re=C bond (1.957( 12) A) (Fig. 4, Table III). " ... 

The dihydrothiazol-2-ylidene (4) was generated by photolysis of matrix-isolated thiazol-2-carboxylic acid.12 Calculations suggested that the barrier to isomerization to thiazole is about 42.3 kcal mol-1 and that the carbene resembles the related imidazol-2-ylidene in structure. An ah initio study of hydroxyoxiranone predicted that the decarboxylation of the zwitterion (5) to form hydroxycarbene (6) would be favourable in vacuo but not in water.13 A theoretical study showed that dihalosulfenes (X2C=SC>2) are best viewed as dihalocarbene-S02 complexes with a carbon-sulfur bond order of approximately zero.14 In a study directed at the elusive thionformic acid (7), tandem mass spectrometric methods were applied to isomeric ethyl thioformates.15 The results suggest that the radical cations generated have the carbene structure [(HS)C(OH)]1. ... 

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