Heterogeneous electron-transfer rate measurements using the scanning electrochemical microscope are reported for the [M(TCTA)](-/0) couples (M = Mn, Fe, and Ni) in aqueous solution. Solution IR spectroscopy indicates that N3O3 coordination is preserved for each couple within the pH range of 2-4, and susceptibility measurements indicate little or no interference from spin-state changes at room temperature. Marcus Hush expressions were used to quantitatively relate structural differences between oxidation states to measured standard heterogeneous electron-transfer rate constants. Good correlation was obtained for the Fe couple, and structural changes associated with the Mn and Ni couples were estimated. In addition, the structure of the Fen complex was determined by X-ray crystallography. The molecule [Fe(H2O)(6)][Fe(TCTA)](2) is trigonal, space group P3(1)/c (no. 159) with a=b=12.530(3) angstrom, c=12.656(4) angstrom, and Z=2. A notable feature of the structure is that the [Fe(TCTA)](-) complex is distributed between two different geometries, one being rigorously trigonal prismatic and the other having a 26 degrees antiprismatic twist.