Four novel low-spin bis(amine) Co(III) porphyrins [Co(TPP)(BzNH(2))(2)] (SbF6), 1, [Co(TPP)(1-BuNH2)(2)](SbF6), 2, [Co(TPP)(PhCH2CH2NH2)(2)](SbF6), 3, and [Co(TPP)(1-MePipz)(2)](SbF6), 4, have been synthesized and characterized by low-temperature X-ray crystallography, IR, electronic, and NMR (H-1,C-13, and Co-59) spectroscopy. The mean Co-N-p distance for the four structures is 1.986(1) Angstrom. The Co-N-ax distances for the 1 degrees amine derivatives average to 1.980(5) Angstrom; the axial bonds of the 2 degrees amine derivative are significantly longer, averaging 2.040(1) Angstrom. The porphyrin core conformation of 4 is significantly nonplanar (mixture of S-4-ruf and D-2d-sad distortions) due to a staggered arrangement of the axial ligands over the porphyrin core and meso-phenyl group orientations < 90 degrees. The X-ray structures have been used with the coordinates for [Co(TPP)(Pip)(2)](NO3) (Scheidt et al. J. Am. Chern. Sec. 1973, 95, 8289-8294.) to parametrize a molecular mechanics (MM) force field for bis(amine) complexes of Co(III) porphyrins. The calculations show that two types of crystal packing interactions (van der Waals and hydrogen bonding) largely control the crystallographically observed conformations. Gas phase conformational energy surfaces have been computed for these complexes by dihedral angle driving methods and augmented with population distributions calculated by MD simulations at 298 K; the calculations demonstrate that the bis(lo amine) complexes are significantly more flexible than the bis(2 degrees amine) analogues. Co-59 NMR spectra have been acquired for a range of [Co(TPP)(amine)(2)]Cl derivatives as a function of temperature. The Co-59 chemical shifts increase linearly with increasing temperature due to population of thermally excited vibrational levels of the (1)A(1) ground state. Activation energies for molecular reorientation (tumbling) have been determined from an analysis of the Co-59 NMR line widths as a function of 1/T; lower barriers exist for the conformationally rigid 2 degrees amine derivatives (2.6-3.8 kJ mol(-1)). The Co-59 chemical shifts vary linearly with the DFT-calculated radial expectation values (3d) for the Co(III) ion. The correlation leads to the following order for the a-donor strengths of the axial ligands: B(z) over dotNH(2) greater than or equal to Cl- > 1-BuNH2 > PhCH2CH2NH2 > 1-Bu2NH > Et2NH. The Co-59 NMR line widths are proportional to the square of the DFT-calculated valence electric field gradient at the Co nucleus. Importantly, this is the first computational rationalization of the Co-59 NMR spectra of Co(III) porphyrins.