A (carbonmonoxy)heme complex with a weak proximal bond. Molecular stereochemistry of...

The synthesis and structural characterization of a six-coordinate (carbonmonoxy)(porphyrin)iron(II), carbonyl(deuteroporphinato)(tetrahydrofuran)iron(II), is described. The choice of tetrahydrofuran as the sixth ligand represents one of the weakest field and weakest binding ligands possible and leads to stronger than usual bonding of CO to the heme. The Fe--C(CO) bond distance is 1.706 A, unusually short. The Fe--O(THF) bond distance in the low-spin complex is 2.127 A. The Fe--C--O and O--Fe--C groups are essentially linear with angles of 178.3 degrees and 177.4 degrees, respectively. The iron atom is displaced out of plane 0.10 A toward the tightly bound CO. The results suggest that the different CO stretching frequencies observed in myoglobin might result from modulating the bonding of the histidine trans to CO. Crystal data are as follows: a = 11.524 (3) A, b = 14.915 (5) A, c = 10.583 (2) A; alpha = 91.51 (2) degrees, beta = 108.99 (2) degrees, gamma = 102.86 (3) degrees; triclinic, space group P1, Z = 2; rho calcd = 1.363 g/cm3, rho obsd = 1.370 g/cm3.