Results are presented from a variable-temperature solution-phase laser photolysis study of the T-5(2) --> (1)A(1) intersystem crossing in a series of related complexes : [Fe(tpen)](ClO4)(2), [Fe(tppn)](ClO4)(2), [Fe(tptn)](ClO4)(2), [Fe(t-tpchxn)](ClO4)(2), and [Fe(dpa)(2)](ClO4)(2). The hexadentate ligands are formed with four 2-pyridylmethyl arms attached to ethylenediamine (tpen), 1,2-diaminopropylene (tppn), 1,3-diaminopropylene (tptn), or trans-1,2-diaminocyclohexane (t-tpchxn). The dpa ligand is a tridentate analogue of these ligands, namely, bis(2-pyridylmethyl)amine. The complex [Fe(mtpen)](ClO4)(2) .(2)/(3) H2O, where mtpen is the same as tpen except one of the pyridyl rings has a 6-methyl substituent, crystallizes in the space group C2/c, which at 173 K has a unit cell with a = 41.390(13) Angstrom, b = 9.5239(16) Angstrom, c = 24.016(6) Angstrom, beta = 108.24(3)degrees, and Z = 12, Refinement with 2844 observed [F > 5.0 sigma(F)] reflections gave R = 0.075 and R(w) = 0.076. The complex [Fe(tppn)](ClO4)(2) .(2)/3H2O crystallizes in the space group P2(1)/n, which at 296 K has a unit cell with a = 12.979(4) Angstrom, b = 12.624(4) Angstrom, c = 19.475(6) Angstrom, beta = 108.17(2)degrees, and Z = 4. Refinement with 2357 observed [F > 5.0 sigma(F)] reflections gave R = 0.1198 and R(w) = 0.1141. The mtpen complex is a high-spin Fe-II complex at all temperatures (4.2-400 K). The hydrated tpen complex is a spin-crossover complex with the temperature where there are 50% high-spin complexes (T-1/2) is similar or equal to 385 K, the hydrated tppn complex is also spin-crossover with a higher T-1/2 value, and the hydrated tptn complex is low spin up to 400 K. The present crystallographic results, together with previously reported structural results for the tpen complex at two temperatures, are used to show that the conversion from low spin to high spin leads to an increase in the trigonal twist of these distorted octahedral complexes. : [Fe(tpen)](ClO4)(2), [Fe(tppn)](ClO4)(2), [Fe(tptn)](ClO4)(2), [Fe(t-tpchxn)](ClO4)(2), and [Fe(dpa)(2)](ClO4)(2). The hexadentate ligands are formed with four 2-pyridylmethyl arms attached to ethylenediamine (tpen), 1,2-diaminopropylene (tppn), 1,3-diaminopropylene (tptn), or trans-1,2-diaminocyclohexane (t-tpchxn). The dpa ligand is a tridentate analogue of these ligands, namely, bis(2-pyridylmethyl)amine. The complex [Fe(mtpen)](ClO4)(2) .(2)/(3) H2O, where mtpen is the same as tpen except one of the pyridyl rings has a 6-methyl substituent, crystallizes in the space group C2/c, which at 173 K has a unit cell with a = 41.390(13) Angstrom, b = 9.5239(16) Angstrom, c = 24.016(6) Angstrom, beta = 108.24(3)degrees, and Z = 12, Refinement with 2844 observed [F > 5.0 sigma(F)] reflections gave R = 0.075 and R(w) = 0.076. The complex [Fe(tppn)](ClO4)(2) .(2)/3H2O crystallizes in the space group P2(1)/n, which at 296 K has a unit cell with a = 12.979(4) Angstrom, b = 12.624(4) Angstrom, c = 19.475(6) Angstrom, beta = 108.17(2)degrees, and Z = 4. Refinement with 2357 observed [F > 5.0 sigma(F)] reflections gave R = 0.1198 and R(w) = 0.1141. The mtpen complex is a high-spin Fe-II complex at all temperatures (4.2-400 K). The hydrated tpen complex is a spin-crossover complex with the temperature where there are 50% high-spin complexes (T-1/2) is similar or equal to 385 K, the hydrated tppn complex is also spin-crossover with a higher T-1/2 value, and the hydrated tptn complex is low spin up to 400 K. The present crystallographic results, together with previously reported structural results for the tpen complex at two temperatures, are used to show that the conversion from low spin to high spin leads to an increase in the trigonal twist of these distorted octahedral complexes.