The X-ray structure of [Et4N][(Ph3Sn)3Cr(CO)4] shows the presence of seven-coordinate chromium bound to four terminal carbonyl groups and three triphenyltin units. The anion is of approximate C3v symmetry; a triangular face of tin atoms is capped by a carbon monoxide unit. This is the first structurally characterized chromium carbonyl complex containing seven unidentate ligands and the initial example of a molecule of the general formula X3Cr(CO)4 -. The crystalline material is triclinic (space group P1) with cell parameters a = 14.371 (4) Å, b = 20.560 (10) Å, c = 10.730 (5) Å, α = 99.21 (4)°, β = 105.07 (3)°, Γ = 83.29 (3)°, V = 3012 (4) Å3, and Z = 2. Reactions of (Ph3Sn)3Cr(CO)4 - with Ph3SnLi, MeLi, Ph3P, and hexamethylphophoramide result in the loss of one triphenyltin group and the formation of moderate to high yields of the six-coordinate anion (Ph3Sn)2Cr(CO)4 2-.