The compounds (η 5-C 5H 5) 2Co 2(NO) 2− x (CO) x ( x = 1, 0) crystallize in space group P2 1/ c with Z = 2 molecules per unit cell. The unit cell constants for the former ( x = 1) are: a 7.878(5), b 6.121(1), c 12.080(4) Å and β 105.46(2)°; while those for the latter ( x = 0) are: a 7.883(1), b 6.117(1), c 12.119(3) Å and β = 105.44(2)°. In both cases the fragment Co-μ 2-(NO) 2− x (CO) x Co is planar, with a maximum deviation of any atom in that fragment from its least squares plane of 0.004 Å. The Cp rings are planar and have normal CC distances. The perpendiculars to the Cp planes make angles of 90° with the normals to the Co(NO) 2− x (CO) x Co planes. In what followes the values given in parentheses refer to the x = 0 derivative, the other values refer to the carbonylnitrosyl derivative ( x = 1). The range of CoC(Cp) distances is 2.071 to 2.103 Å, mean 2.088 Å (2.086 to 2.115 Å, mean 2.101 Å). The Cp ring CC distances range from 1.379 to 1.401 Å, mean 1.388 Å (1.381 to 1.445 Å, mean 1.411 Å). The CoN (or C) distances are 1.829 and 1.831 Å (1.824 and 1.827 Å). The NCoN (or C) angle is 99.3° (99.0°). The two independent values of the CoN (or C)O angles are 139.5 and 139.8° (139.4 and 139.6°) while the value of the CoN(or C)Co angle is 80.7° (81.1°). The CoCo distance is 2.370 (2.372) Å. The EAN rule is discussed with regard to these and related observations. The paramagnetic carbonylnitrosyl derivative gives a 15 line ESR spectrum (room temperature, benzene) of which the lines have, approximately, intensity ratios of 1/2/3⋯7/8/7⋯3/2/1, expected for an unpaired electron distributed equally over two 59Co nuclei. The 59Co hyperfine splitting is 47.4 Oe, the g value is 2.0539 and the linewidth is ca. 29 Oe. At room temperature there is no evidence of a 14N hyperfine splitting from the bridging nitrosyl.