The addition of the strongly pi-bonding ligands CO or tert-butyl isocyanide to the low-spin five-coordinate iron(II) nitrite species [Fe(TpivPP)(NO2)](-) (TpivPP = picket fence porphyrin) gives two new six-coordinate species [Fe(TpivPP)(NO2)(CO)](-) and [Fe(TpivPP)(NO2)(t-BuNC)](-). These species have been characterized by single-crystal structure determinations and by UV-vis, IR, and Mossbauer spectroscopies. All evidence shows that in the mixed-ligand iron(II) porphyrin species, [Fe(TpivPP)(NO2)(CO)](-), the two trans, pi-accepting ligands CO and nitrite compete for pi density. The CO ligand however dominates the bonding. The Fe-N(NO2) bond lengths for the two independent anions in the unit cell at 2.006(4) and 2.009(4) Angstrom are lengthened compared to other nitrite species with either no trans ligands or non-pi-accepting trans ligands to nitrite. The Fe-C(CO) bond lengths are 1.782(4) Angstrom and 1.789(5) Angstrom for the two anions. The two Fe-C-O angles at 175.5(4) and 177.5(4)degrees are essentially linear in both anions. The quadrupole splitting for [Fe(TpivPP)(NO2)(CO)](-) was determined to be 0.32 mm/s, and the isomer shift was 0.18 mm/s at room temperature in zero applied field. Both of the Mossbauer parameters are much smaller than those found for six-coordinate low-spin iron(II) porphyrinates with neutral nitrogen-donating ligands as well as iron(II) nitro complexes. However, the Mossbauer parameters are typical of other six-coordinate CO porphyrinates signifying that CO is the more dominant ligand. The CO stretching frequency of 1974 cm(-1) is shifted only slightly to higher energy compared to six-coordinate CO complexes with neutral nitrogen-donor ligands trans to CO. Crystal data for [K(222)][Fe(TpivPP)(NO2)(CO)](-) 1/2C(6)H(5)Cl: monoclinic, space group P2(1)/c, Z = 8, a = 33.548(6) Angstrom, b = 18.8172(15) Angstrom, c = 27.187(2) Angstrom, beta = 95.240(7)degrees, V = 17091(4) Angstrom(3).