Reactions without metal-ligand bond breaking

The tetradentate ligand forms monomeric square planar complexes. Synthetic and kinetic studies reveal that the coordinated mercapto group may be converted into the coordinated thioether function without breaking the metal-sulfur bond. The nucleophilic power of the coordinated mercapto group exceeds that of RSH, but depends on the metal atom. Bridging protects the sulfur atom from alkylation. In the case of nickel(ll), alkylation is accompanied by expansion of the coordination number of the nickel from 4 to 6. Ligand reactions have led to the synthesis of planar ligands completely cydized about the metal ion. 

The metal ion does, however, introduce a new subtlety into these reductions. The reduction of the two imine groups in the nickel(n) complex 4.10 is readily achieved with Na[BH4], The free tetraamine ligand would be expected to exhibit a facile pyramidal inversion at each nitrogen atom, whereas in the nickel(n) complex this inversion is not possible without significant weakening (or breaking) of the Ni-N bonds. In macrocyclic complexes it is very often found that the complex obtained by the reduction of a co-ordinated imine does not possess the same stereochemistry as that obtained by the direct reaction of the free amine with metal ion. 

Inner sphere oxidation-reduction reactions, which cannot be faster than ligand substitution reactions, are also unlikely to occur within the excited state lifetime. On the contrary, outer-sphere electron-transfer reactions, which only involve the transfer of one electron without any bond making or bond breaking processes, can be very fast (even diffusion controlled) and can certainly occur within the excited state lifetime of many transition metal complexes. In agreement with these expectations, no example of inner-sphere excited state electron-transfer reaction has yet been reported, whereas a great number of outer-sphere excited-state electron-transfer reactions have been shown to occur, as we well see later. 

A kinetic study of the alkylation of coordinated thiol in NH2CH2-CH2SH and related ligands revealed that the reaction proceeded without the breaking of the metal-sulfur bond (11). The nucleophilicity of the coordinated sulfur was found to depend on the central metal involved but was greater than that of the sulfur in a thiol. It was found that sulfur in such ligands did not react with alkyl halides when it occupied a bridging position between two metal atoms in a polynuclear complex. 

Ligands play two basic roles in homogeneous catalysts. They help to stabilize metal complexes without taking part in the bond breaking, formation, or reorganization process during catalysis. Alternatively, on coordination to a metal atom, they are activated and take part in the catalytic reaction as one of the reactants. 

Abstract A variety of structural types of transition metal complexes containing a-borane ligands are reviewed. Their structure and bonding are discussed. Compared with other types of a-complexes, a-borane complexes display quite different structural and bonding properties because of the electron unsaturated boron center in the a-borane ligands in the precoordination state. The availability of an sp3-hybridized orbital at the boron center allows stronger back-donation interaction without breaking the coordinated B-H bond. The role played by a-borane complexes in hydroboration and borylation reactions has also been reviewed. 

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