Neutral ligands, charge separation

Figure 22.7 Components of a coordination compound. Coordination compounds, shown here as models (top), perspective drawings (middle), and chemical formulas (bottom), typically consist of a complex ion and counter ions to neutralize the charge. The complex ion has a central metal ion surrounded by ligands. A, When solid [Co(NH3)6]Cl3 dissolves, the complex ion and the counter ions separate, but the ligands remain bound to the metal ion. Six ligands around the metal ion give the complex ion an octahedral geometry. B, Complex ions with a central d metal ion have four ligands and a square planar geometry.

Since the porphyrin ring neutralizes only two of the positive charges of the metal, Fe porphyrins must have an associated counterion. This counterion may be directly bound to the metal, as in five-coordinate high-, intermediate-, and low-spin Fe porphyrins, or may exist as a separate species that forms an ion-pair with the iron porphyrin complex, as in all six-coordinate complexes of Fe porphyrins with neutral ligands. We will survey the structures, properties, and reactions of Fe porphyrins of each of the possible spin states. 

This makes it difficult to assign clearly separated formal charges to the metal and to the ligand, which would not be meaningful as a consequence of the noninnocent nature of 1,2-dithiolene ligands.53,54 In Scheme 5, the case of neutral diamagnetic square-planar Zw(l,2-dithiolene) complexes is illustrated. 

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