A lo7Au M b b a u e r Study of a Series of Isocyanide, Carbene, or Methanide Derlvatives of Gold. The Crystal Structure of trans ,trans-and cis ,cis-[ (Carbene),Au]CIO, and of cis ,trans-(Carbene)AuCI (Carbene = p-MeC,H,NHCOEt) 1Q7Miissbauer spectra were recorded for a homogeneous series of two-coordinate organogold(1) and four-coordinate organogold(II1) complexes: LAuC1, [L,Au]+ (where L may be Ar-NC or a carbene such as (Ar-NH)J! or (Ar-NH)(EtO)C, Ar being p-tolyl), [L2Au12]+ (where L is a carbene), [(Ar-N=)-(EtO)CAuI3, and [(Ar-N=)(Ar-NH)C-Au]..The Mossbauer parameters are dominated by the Q donor properties of the ligands used; they are sensitive to the immediate environment of the gold atom and not to the geometrical isomerism found in the carbene derivatives investigated. Such isomerism, detected in solution by NMR, was confirmed by X-ray crystal structure determinations carried out on the trans,trans (7tt) and cis,& (7cc) steric isomers of ([ (p-MeC6H4NH)(EtO)C]2Au}C104 (7, Cz&&06C1Au) as well as on another isomer, (cis,trans-MeC6H4NH)(EtO)CAuC1 (5, CloH,3NOC1Au (5ct)). Compounds 7tt, 7cc, and 5ct crystallize in the triclinic system of space group P1. 7tt: a = 11.686 (3) A, b = 11.257 (4) A, c = 11.258 (4) A, a = 112.5 (ti)', p = 102.5 (4)O, y = 112.2 (6)", Z = 2. 7cc: a = 13.493 (5) A, b = 9.283 (6) A, c = 9.978 (5) A, a = 94.0 (4)O, 6 = 107.4 (3)O, y = 99.6 (3)O, Z = 2. 5ct: a = 10.000 (3) A, b = 12.953 (4) A, c = 9.680 (3) A, CY = 90.9 ( 4 ) ' , = 108.0 (4)O, y = 88.8 (3)O, 2 = 4. Comparison between Mossbauerspectra, NMR spectra, and X-ray crystal structures reveals some limitation of the techniques employed when each alone is considered, while these together provide a satisfactory characterization and suggest that the amount of Au-C ?r bonding in our compounds is negligible.