The effect of a sixth ligand in a series of low-spin thiocarbonyl-ligated iron(II)octaethylporphyrinates has been investigated. Six-coordinate complexes have been synthesized and characterized by Mossbauer and infrared spectroscopy and single-crystal X-ray structure determinations. The results are compared with the five-coordinate parent complex. The crystal structures of [Fe(OEP)(CS)(1-Melm)] and [Fe(OEP)(CS)(Py)] are reported and discussed. The 1-methylimidazole and pyridine derivatives exhibit Fe-C(CS) bond distances of 1.703(4) and 1.706(2) A that are significantly longer than the 1.662(3) Angstrom reported for five-coordinate [Fe(OEP)(CS)] (Scheidt, W. R.; Geiger, D. K. Inorg. Chem. 1982, 21, 1208). The trans Fe-N(ligand) distances of 2.112(3) and 2.1550(15) A observed for the 1-methylimiclazole and pyridine complex are similar to0.13 Angstrom longer than those observed for analogous bis-ligated complexes and are consistent with a significant structural trans effect for the CS ligand. Mossbauer investigations carried out for five- and six-coordinate thiocarbonyl derivatives with several different sixth axial ligands reveal interesting features. All derivatives exhibit very small isomer shift values, consistent with a very strong interaction between iron and CS. The five-coordinate derivative has delta(Fe) = 0.08 mm/s, and the six-coordinate complexes exhibit delta(Fe) = 0.14 to 0.19 mm/s at 4.2 K. The five-coordinate complex shows a large quadrupole splitting (DeltaE(q) = 1.93 mm/s at 4.2 K) which is reduced on coordination of the sixth ligand (DeltaE(q) = 0.42-0.80 mm/s at 4.2 K). Addition of a sixth ligand also leads to a small decrease in the value of nu(CS). Correlations in structural, IR, and Mossbauer results suggest that the sixth ligand effect is primarily induced by changes in or-bonding. The structure of [Fe(OEP)(CS)(CH3OH)] is briefly reported. Crystal data: [Fe(OEP)(CS)(1-Melm)] crystallizes in the monoclinic system, space group P2(1)/n, Z = 4, a = 9.5906(5) Angstrom, b = 16.704(4) Angstrom, c = 23.1417(6) Angstrom, beta = 100.453(7)degrees. [Fe(OEP)(CS)(Py)] crystallizes in the triclinic system, space group P1, Z = 5, a = 13,9073(6) Angstrom, b = 16.2624(7) Angstrom, c = 22.0709(g) Angstrom, alpha = 70.586(1)degrees, beta = 77.242(1)degrees, gamma = 77.959(1)degrees. [Fe(OEP)(CS)(CH3OH)] crystallizes in the triclinic system, space group P1, Z = 1, a = 9.0599(5) Angstrom, b = 9.4389(5) Angstrom, c = 11.0676(6) Angstrom, alpha = 90.261(1)degrees, beta = 100.362(1)degrees, gamma = 114.664(1)degrees.