Point five-coordinate complexes

One final point just as we found that the number of unpaired electrons in an octahedral complex can vary with the crystal field, so, too, in other geometries. However the effects tend to be more subtle and difficult to disentangle. Thus, for five-coordinate complexes they are interwoven with the trigonal bipyramidal and square pyramidal structural possibilities. ... 

CH3Co" Pc (Pc = phthalocyanine dianion) has been characterized by equilibrium studies of its /raar-axial ligation and cyclic voltammetry as a relatively electron-poor model of methyl cobalamin, which, in non-coordinating solvents, remained a five-coordinate complex (Scheme 18). Axial base inhibition of methyl transfer from CH3Co" Pc showed that the reaction proceeded via a reactive five-coordinate species, even in coordinating solvents. The virtual inactivity of six-coordinate CH3Co" Pc(L) complexes provided a reference point for important biological processes. ... 

An intermediate spin state (a quartet 4A2) similarly is feasible for five-coordinate iron(III) though, as pointed out by Kahn [118], the situation may be more complex. If the states are close in energy then they can interact through spin-orbit coupling to give a so-called spin-admixed ground state. 

Mechanistic interpretation of activation volumes on square-planar complexes is complicated by the geometry. The sterically less crowded complexes may have loosely bound solvent molecules occupying the axial sites above and below the plane. Replacing them in the formation of a five-coordinate transition state or intermediate may result by compensation in relatively small volume effects. It is therefore difficult to distinguish between Ia and A mechanisms from the value of the activation volume. Nevertheless, the AV values are negative and together with the second-order rate laws observed, point to an a-activation for those solvent exchange reactions. 

Dr. Halpern This could be used in stabilizing, say an activated complex. The point about the hydrolysis observation is that this refers to the octahedral complex, whereas the explanations that have been offered for the effect of amide in the conjugate base mechanism are concerned, not with weakening of the binding, but with stabilizing a five-coordinated intermediate. I wondered if the role of the hydroxide in promoting water substitution might be of the same nature. 

From this point of view, some interesting metalloporphyrins have been produced by clever synthetic techniques. Collman etall0l) synthesized the Fe(II) complex of picket-fence porphyrin 42 to favor the five-coordinate structure and inhibit dimerization. Their porphyrin derivative had steric bulkiness on one side of... 

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