Nickel complexes, metal exchange reactions

The problem of the origin of the metal-porphyrins is closely related to that of the origin of petroleum and is oie of the most basic and interesting questions of petroleum geochemistry. The most probable conclusion seems to be that the nickel and vanadium porphyrin complexes are formed by metal exchange reactions from animal and/or plant metabolic pigments such as hemoglobin and chlorophyll. 

Excellent examples of metal exchange reactions are provided by complexes of tetraphenylcyclobutadiene. When either iron pentacarbonyl or nickel carbonyl is reacted with tetraphenylcyclobutadiene palladium bromide, the corresponding metal is exchanged with the release of palladium(O) metal. The reaction proceeds only in aromatic solvents and seems to be generally applicable to a variety of transition metal carbonyls. As illustrated in Fig. 6-8, the first step is proposed to involve the formation of uncomplexed tetraphenylcyclobutadiene, a mixed metal halide carbonyl, carbon monoxide. 

A transmetalation of the styrylcarbene chromium complex 62 in the presence of stoichiometric amounts of [Ni(cod)2] to give the nickel carbene intermediate 63 was applied to the synthesis of Cr(CO)3-coordinated cycloheptatriene 64 upon reaction with terminal alkynes [57] (Scheme 37). The formation of pen-tacarbonyl(acetonitrile)chromium is expected to facilitate the metal exchange. 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

Dale Margerum Ralph Wilkins has mentioned the interesting effect of terpyridine on the subsequent substitution reaction of the nickel complex. I would like to discuss this point—namely the effect of coordination of other ligands on the rate of substitution of the remaining coordinated water. However, before proceeding we should first focus attention on the main point of this paper-which is that a tremendous amount of kinetic data for the rate of formation of all kinds of metal complexes can be correlated with the rate of water substitution of the simple aquo metal ion. This also means that dissociation rate constants of metal complexes can be predicted from the stability constants of the complexes and the rate constant of water exchange. The data from the paper are so convincing that we can proceed to other points of discussion. 

The complications which result from the hydrolysis of alkali metal cyanides in aqueous media may be avoided by the use of non-aqueous solvents. The one most often employed is liquid ammonia, in which derivatives of some of the lanthanides and of titanium(III) may be obtained from the metal halides and cyanide.13 By addition of potassium as reductant, complexes of cobalt(O), nickel(O), titanium(II) and titanium(III) may be prepared and a complex of zirconium(0) has been obtained in a remarkable disproportion of zirconium(III) into zirconium(IV) and zirconium(0).14 Other solvents which have been shown to be suitable for halide-cyanide exchange reactions include ethanol, methanol, tetrahydrofuran, dimethyl sulfoxide and dimethylformamide. With their aid, species of different stoichiometry from those isolated from aqueous media can sometimes be made [Hg(CN)3], for example, is obtained as its cesium salt form CsF, KCN and Hg(CN)2 in ethanol.15... 

Some homoleptic unsymmetrical (dmit/mnt, dmit/tdas) dithiolene nickel complex-based D-A compounds with D = TTF and EDT-TTF also exhibit metal-like conductivity (see Table I) (101). Their molecular structure is shown in Scheme 3. The unsymmetrical tetraalkylammonium salts [MLjLJ- (M = Ni, Pd, Pt) have been prepared by ligand exchange reaction between tetraalkylammonium salts of MLj and ML21 (128, 129) and the D-A compounds have been synthesized by electrooxidation. Among these complexes, only the Ni derivatives exhibit metallic-like properties, namely, TTF[Ni(dmit)(mnt)] (metallic down to --30 K), a-EDT-TTF[Ni(dmit)(mnt)] (metallic down to 30 K), TTF[Ni(dmit)(tdas)] (metallic down to 4.2 K), and EDT-TTF[Ni(dmit)(tdas)] (metallic down to --50 K) (see Table I). The complex ot-EDT-TTF-[Ni(dmit)(mnt)J is isostructural (130) to a-EDT-TTF[Ni(dmit)2)] [ambient pressure superconductor, Section II.B.2 (124)]. Under pressure, conductivity measurements up to 18 kbar show a monotonous decrease of the resistivity but do not reveal any superconducting transition (101). 

The nickel ) complex of 92 cannot be prepared directly via the template method, but can be prepared by a transmetallation procedure. Synthesis of the macrocycle in the presence of one of the metal ions known to be effective as a template is followed by a metal exchange process in solution to insert the nickel ) ion. This cation exhibits a strong preference for the square planar, square pyramidal, and octahedral geometries 79). Thus the failure of the nickel ) cation to behave as a template ion in the synthesis of 92 is probably due to the disinclination of the metal to accommodate the pentagonal array of donor nitrogen atoms necessary for reaction to occur. 

The preparations described here are developed from published work by Malatesta et al.5 and from more recent studies in the contributors own laboratory.2 The cobalt and nickel complexes are prepared by reduction of the corresponding metal nitrates with sodium tetrahydroborate in the presence of excess ligand, whereas the syntheses of the rhodium and platinum complexes involve simple ligand exchange processes. The preparative routes are suitable for use with triphenyl- or p-substituted triphenyl phosphites reactions involving o- or m-substituted triphenyl phosphites give much reduced yields of products which are difficult to crystallize and are very air-sensitive. These features probably reflect the unfavorable stereochemistry of the o- and m-substituted ligands. 

This square planar zinc complex is an excellent intermediate for the preparation of other BZO4 [16] octaene N4 complexes by metal exchange, and its use alleviates the problem of separating the desired complex from other reaction side products when o-aminobenzaldehyde is condensed in the presence of the appropriate metal ion. Complexes of nickel, cobalt, iron, and palladium have been prepared from this zinc compound. 

Chattopadhyay et al [Ch 76a, Ch 76b, Ch 77] used a spectrophotometric stopped flow technique to study the kinetics of formation and dissociation of a large number of nickel complexes, with a metal to ligand ratio of 1 1, in various non-aqueous solvents. When their findings were compared with the results obtained earlier in investigations of analogous complexes of other transition metal ions [Ch 73, Ch 74], it was established that in most of the systems the above reaction follows an /d mechanism. The step determining the reaction rate was found to be the exchange between the solvent bound in the coordination sphere of the metal and the... 

The substitution of CO in metal carbonyls by olefinic and acetylenic compounds is one of the chief methods for preparing tt complexes of transition metals. Unfortunately this procedure fails almost completely when applied to nickel carbonyl, and this may be one of the reasons why until recently no tt complexes of nickel with olefinic or acetylenic ligands were known. The reasons for this behavior of nickel carbonyl will become clearer, if both its electronic structure and the mechanism of the ligand exchange reactions are considered. 

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