Reduction of co-ordinated ligands

A few studies have appeared dealing with substitutions brought about by oxidation and reduction of co-ordinated ligands. Mention has already been made in this section of the reactions of co-ordinated azides, amides, and isocyanates with NO+. The reverse process (attack at co-ordinated NO+) is postulated to have a reactivity pattern which correlates with the i.r. N—O stretching frequencies. Co-ordinated NO+ behaves as an electrophile if the N—O stretching frequency is less than 1886 cm and the force constant is less than 13.8 mdyn A . A study of the rate of reaction of cis- and /ra -[Co(en)2(N02)2]+ ions with hydrazoic acid has also appeared ... 

The reduction of co-ordinated oxygen by organic substrates has been investigated. Oxygen adducts of 3-methoxysalen-Co in pyridine are formed reversibly, and reaction takes place with substrates (RHg) such as thiols, ascorbic acid, and AW -tetramethyl-/ phenylenediamine (tmpd). Since the latter has no significant ligand properties and Wurster s blue radical cation is formed in the course of reaction as confirmed by e.s.r. and u.v., it is suggested that the reaction proceeds without... 

The benefits of C2-symmetry are due to the reduction of the number of possible catalyst-substrate isomers that may form, and the number of enantiospecific pathways (Helmchen, Pfaltz [9]). As they reasoned, there is no fundamental reason why a C2-symmetric ligand is superior qualitate qua to a Ci-symmetric ligand. Two different co-ordinating ligands (P-X as shown above, or P-N, discussed here) could lead to more control, as the attack may... 

The redox nature of co-ordination complexes with macrocyclic ligands has been examined using the trans- A-d CD system. The electrolytic reduction of the copper(n) complex of macrocyclic 8chiflf bases of the type trans- A d QnQ (or tet-a) yields the highly reactive yellow cuprous complex... 

Hughes, D.L., Mohammed, M.Y. and Pickett, C.J. Electroreduction of co-ordinated cyanide to the aminocarbyne ligand (CNH2) and a pathway for isomerisation of ligating methyleneamide (NCH2) Reactions at molybdenum of relevance to cyanide reduction by nitrogenase J.Chem.Soc., Chem. Commun., (1989), 1399-1400... 

Preliminary stopped-flow studio indicate the presence of several short-lived intermediates. The corresponding cobalt(iii) complex, however, yields (quantitatively) CrNO + and Co. An examination of these data thus provides three possible reaction pathways for the reaction of this co-ordinated ligand with a conunon reductant. 

The expected order for electrophilic attack by OH on co-ordinated and unco-ordinated pyridine derivatives is py>[Co(NH3)spy] +>Hpy+. For pyridine and isonicotinamide (ina) this order is observed and similarities in the spectra of the unstable addition products suggest that OH attacks at the same place in both bound and unbound ligands. With nicotinamide (na), reaction of the cobalt(iii) complex is faster (2.1 x 10 1 moh s at 22 C) and produces a transient spectrum which is red-shifted compared with that of the unbound ring adduct. It is suggested that in this case attack occurs not on the aromatic system but on an amide group. This behaviour may explain why the europium(ii) reduction of [Co(NH3)5(ina)] + is autocatalytic whereas the corresponding reduction of the nicotinamide complex is not. In the pulse radiolysis studies, the transient intermediate complexes decay by a second-order pathway with rate constants 1.3 x 10, 3.0 x 10 , and 6.0 x 10 1 mol for [CofNHa) sPy] , [Co(NH3) s(na)] +, and [Co(NH3)5(ina)] +respectively... 

A further example of attack at a co-ordinated ligand involves the two-electron reduction of (5) to give a nitrogen-oxygen ( steine-chelate (6) as well as the cystine dimer (7). ... 

D. L. Toppen and R. G. Linck (Inorg. Chem., 1971,10, 2635) have recently discussed the role of co-ordinated water as a bridging ligand in redox systems and have compared the relative rates of reduction of various complex ions with Cr" and V". For outer-sphere reactions, it appears that fc(Cr")VAc(V") a 0.02. 

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