Tetrazole and triazole carbene complexes of group 6 metal carbonyls

In order to study the relative stability of cis- and trans-isomers of bis(NHC)tetracarbonyl complexes of group 6 metals, we synthesized the corresponding complexes with triazolin- and tetrazolinylidene ligands. By reaction of the free carbene (L = 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazolin-5-ylidene) – first synthesized by Enders – with the hexacarbonyls of Cr, Mo and W the corresponding M(L)(CO)5 complexes are generated. Depending on an excess of carbene also the cis-(L)2Mo(CO)4 complex was obtained. The latter can be photolytically converted to the trans-(L)2Mo(CO)4 complex. The corresponding complexes with the 1,4-dimethyltetrazolin-5-ylidene ligand (L′), Cr(L′)(CO)5, cis-(L′)2Cr(CO)4 and trans-(L′)2Cr(CO)4 can be obtained by reaction of hexacarbonyl-μ-trihydroxy-dichromate with dimethyltetrazolium salt. In the cis-(L′)2Cr(CO)4 complex, one carbonyl ligand can be replaced by donor ligands such as pyridine or phenylisocyanide to form sym–mer-tricarbonyl complexes. All new complexes are fully characterized by spectroscopy and most by single-crystal X-ray analysis.