The structure and the absolute configuration of (-)s79-(75-C5H5)Fe(CO)[P(C6H5)3]COOC10H19 (C10H19 = menthyl) have been determined by single-crystal x-ray diffraction methods using data acquired by a computer-controlled diffractometer and Bijvoet's technique. The substance crystallizes in the orthorhombic system, space group P2,2121, with cell constants of a = 11.221 (4), b = 14.817 (7), and c = 18.958 (3) Á. The observed and calculated densities are 1.25 (2) and 1.25 g/cm3 (for Z = 4 molecules/unit cell). The Fe-(i)5-C5H5) fragment has normal distances and angles and the Cp ring is planar, as expected. The Fe-P(C6H|)3 group has stereochemical parameters which are well within the values found in the literature (i.e., Fe-P = 2.214 (2) A, P-C = 1.830 (6), 1.838 (4), and 1.853 (6) Á), and the dihedral angles of the three phenyl rings are canted at arbitrary values dictated by packing. The most important features of the stereochemistry of the molecule are the following: (1) the Fe-C(C=0) and C=0 distances are, respectively, 1.670 (7) and 1.206 (9) Á. . The former is the shortest iron to carbonyl carbon distance observed thus far. (2) The Fe-ester fragment shows a large angular distortion at the C atom such that Fe-C=0 = 132.1 (5), Fe-C-OC10Hi9 = 117.7 (4), and 0=C-O = 108.8(5)°. Concomitant with this distortion is a shortening of the Fe-C(ester) bond to 1.825 (6) Á. Both of these observations point to an enhanced r-bonding interaction between the iron atom and the carbons of the carbonyl and ester group. When the current rules for ranking groups are applied, the configuration at the iron atom is specified to be 5.