Metal ligand cleavage

The pathway corresponding to heterolytic metal ligand cleavage can be evaluated by suppressing the homolysis reaction... 

The hydrolysis of ethylene phosphate in hydroxide ion solution proceeds with a rate constant of 5 x 10" L-mol" s" (100). The O-P-0 angle in the ring of ethylene phosphate of 99° is expected to be rather similar to that for the four-membered ring incorporating the cobalt and phosphorus centers. Therefore it is likely that reaction at the strained P center of the complex is eclipsed by a more rapid metal-ligand cleavage reaction. This problem can be circumvented by the use of metal ion complexes of Ir(III) where the metal-ligand bonds are more inert as the locus for the reaction. 

As mentioned in Sect. 2.3, cleavage of the P-B bond is required prior to com-plexation with a transition-metal. The following section provides an example for the preparation of two rhodium complexes. Such synthesis, however, may be generalized to almost any other transition-metal/ligand complexes. 

The hexakis(methyl isocyanide) dimers, [Pt2(CNMe)6], undergo photolytic cleavage of the Pt—Pt bond to give 15-electron radicals, Pt(CNMe)3.94 Mixtures of platinum and palladium dimers give rise to heteronuclear complexes under photolytic conditions. Mixtures of normal and deuterium-labeled methyl isocyanide complexes reveal that the metal-ligand bonds undergo thermal redistribution.94... 

M[pz(A4)] A = S2ML2. The octakis(.V-R)porphyra/,ines reported by Schramm and Hoffman (2), M[pz(S-R)8 (M = Ni, Cu), (60), can be converted to the octathiolate M[pz(S )g] (Scheme 11) via reductive cleavage of the sulfur-carbon bond when R = benzyl (Bn), and this tetra-bis(dithiolate) can then be peripherally capped with metal-ligand systems to yield peripherally tetrametalated star porphyrazines. The benzyl dinitrile 57 can be macrocyclized around magnesium butoxide to form [Mg[pz(S-Bn)8] (58) (35-40%), which can then be demetalated with trifluoroacetic acid to form 59 (90%), which is subsequently remetalated with nickel or copper acetate to form 60a (95%) and 60b (70%) (Scheme 11) (3, 23, 24). Deprotection of 60a or 60b with sodium in ammonia yields the Ni or Cu tetra-enedithiolates, 61a or 61b to which addition of di-ferf-butyl or n-butyl tin dinitrate produces the peripherally metalated star porphyrazines 62a (37%), 62b (80%), and 62c (41%). 

DR. JAMES ESPENSON (Iowa State University) I should like to raise a slightly different aspect of substitution reactions, an aspect which relates to a different kind of mechanism. This involves a change from the conventional heterolytic process of metal ligand bond cleavage to one dealing with homolytic bond cleavage. 

The second type of system in which heterolytic cleavage is favored arises with a metal-ligand complex having an atomic metal ion (with a s°dI1+1 configuration) and a two electron donor, L 2. A prototype is (Ag C6H6)+ which was observed to photodissociate to form X + Y = Ag(2S, s M10) + C6H6+(2B j) rather than the lower energy (heterolytically cleaved) dissociation limit Y + X2 =... 

Electrochemically Induced Metal-ligand Bonds Cleavage. 568... 

Electrochemically Induced Metal-Ligand Bonds Cleavage. 577... 

The cleavage of one equatorial Mo—X or Mo—S bond upon one-electron reduction of 2, 3, and 4+ was assigned to unfavorable electrostatic interactions between the lobes of the half-filled Mo 4dxy antibonding orbital and the five equatorial metal-ligand bonding electron pairs [23]. This interpretation was confirmed by the fact that the Mo(VI)/Mo(V) reduction of six-coordinate complexes (i.e. with four equatorial ligands) was found fully reversible, as expected [25]. 

Electrochemical cleavage of metal-ligand bond(s) is a convenient way of generating vacant coordination sites at a metal center (see Sch. 5). The eliminated ligand can be neutral (L) or anionic (X). 

For an X—Y substrate, activation in this manner can ultimately lead to cleavage of the X—Y bond in a process called oxidative addition (5, 6, 16). The metal complex center has undergone an increase in both coordination number and oxidation state since in this formalism the electron pairs in metal-ligand bonds are associated with the ligands (14, 15). While substrate activation by oxidative addition occurs in this way for H2 (16), the term oxidative addition really represents a stoichiometric transformation, and does not necessarily imply a specific mechanism. In fact, studies over the past decade have shown that the interaction of X—Y with a metal center to give X—M—Y proceeds by any of a variety of mechanisms determined by the substrate and the metal complex (16-18). However, once the X—M—Y species is formed, the X—Y substrate can be viewed as activated. 

The weakness of most metal-metal bonds compared with metal-ligand bonds makes cleavage of metal-metal bonds by nucleophiles a common process (15). In the case of metal-metal double bonds this corresponds to nucleophilic addition to the metal-metal bonded systems. Since unsaturated clusters exist which can be considered to contain metal - metal double bonds, this should be an important aspect of substrate activation by clusters. 

---