Electronic Structure and Electron Transport Properties of Metal Ions 1 1 



appears as methyl alcohol attached to the Cr'++ ion (Taube, 1959). This 

 implies the following mechanism: 



OMe OMe 



O— Cr++(H.,0)5 O— Cr+++(H.>0)5 



t " t 



Weak bond Strong bond 



OMe O 



\v_C > ^ 



\.. transfer 



c 



\ 



O— Cr++^(HoO)5 O— Cr(H20)4(MeOH2)+ 



(ii) 



O— Cr+^+(H20)4(MeOH) + H+ 



In this system the reaction achieved is hydrolytic, that is, the redox reaction 

 catalyses stoichiometrically the hydrolysis of the ester, but I shall now show 

 that this is not an essential feature of the process. In order to bring the 

 discussion closer to the subject of the cytochromes, I shall replace the 

 reactants by ones which may be biologically important, but only for illustrative 

 purposes : 



PO,H- 

 / A 



Fe+++— O— (/ \)— O ^ 



\ Fe++ADP 



Weak bond 



Fe++— O— (/ \>— O 



PO.H- 



t Fe+++— ADP 



strong bond 



^-^ > Fe++— O— <f \— O (iii) 



(r03+) transfer \ / \ 



Fe+++ATP 



The essential feature of the reaction is that the redox energy is preserved 

 because a ligand which normally appears only loosely associated with metal 

 ions, owing to redox-equilibrium A, finds itself in a position normally 

 occupied only by strongly co-ordinating ligands. Under such circumstances 

 the weakly co-ordinating ligand undergoes fission to give a much more stable 



H.E. — VOL. I — c 



