(η^5-1-carboethoxy-2-methylcyclopentadienyl)dicarbonylnitrosylchromium, (η^5-1-carboethoxy-3-methylcyclopentadienyl)dicarbonylnitrosylchromium, and dicarbonyl(η^5-1-carboxy-2-methylcyclopentadienyl)nitrosylchromium [η^5-2-(CH_3)C_5H_3COOR]Cr(CO)_2(NO) ( R = H, C_2H_5) and [η^5-3-(CH_3)C_5H_3COOC_2H_5]Cr(CO)_2(NO)

Reaction of sodium methylcyclopentadienide with diethylcarbonate led to a mixture of sodium 1-carboethoxy-2-methylcyclopentadienide and sodium 1-carboethoxy-3-methylcyclopentadienide. After the mixture was refluxed with Cr(CO)6 in DMF, the metal carbonyl anion obtained was converted to the corresponding hydrido complexes [η^5-2-(CH_3)C_5H_3COOC_2H_5]Cr(CO)_3(H) and [η^5-3-(CH_3)C_5H_3COOC_2H_5]Cr(CO)_3(H), by adding acetic acid. A subsequent nitrosylation with N-methyl-N-nitroso-p-toluenesulfonamide gave the corresponding ester complexes 3 and 4. Chromium acid 5 was obtained after saponification of 3.The chemical shifts of C(2,5) and C(3,4) on the Cp ring of 3-5 have been assigned using two-dimensional HETCOR NMR spectroscopy.