The Central Metal Ion

From experimental evidence, the first row transition metals clearly prefer oxidation states +11 and -Fill. Higher and lower formal oxidation states, even formally negative ones (such as —I, —II), are known, although oxidation states less than +1 are almost exclusively 

Introduction to Coordination Chemistry Geoffrey A. Lawrance 2010 John Wiley Sons, Ltd 

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When naming complex ions the number and type of ligands is written first, followed by the name of the central metal ion. If the complex as a whole has a positive charge, i.e. a cation, the name of the central metal is written unchanged and followed by the oxidation state of the metal in brackets, for example [Cu(N 113)4] becomes tetra-ammine copper(II). A similar procedure is followed for anions but the suffix -ate is added to the central metal ion some examples are ... 

The solid anhydrous halides of some of the transition metals are often intermediate in character between ionic and covalent their structures are complicated by (a) the tendency of the central metal ion to coordinate the halide ions around it, to form an essentially covalent complex, (b) the tendency of halide ions to bridge, or link, two metal ions, again tending to covalency (cf. aluminium chloride, p. 153 and iron(III) chloride, p. 394). 

Fig. 6. Hydrated sodium ion,, in aqueous solution (4).The H2O molecules form ion—dipole bonds to the central metal ion. The waters are in...

When a Br nsted base functions catalytically by sharing an electron pair with a proton, it is acting as a general base catalyst, but when it shares the electron with an atom other than the proton it is (by definition) acting as a nucleophile. This other atom (electrophilic site) is usually carbon, but in organic chemistry it might also be, for example, phosphorus or silicon, whereas in inorganic chemistry it could be the central metal ion in a coordination complex. Here we consider nucleophilic reactions at unsaturated carbon, primarily at carbonyl carbon. Nucleophilic reactions of carboxylic acid derivatives have been well studied. These acyl transfer reactions can be represented by... 

As you can deduce from the preceding examples, the oxidation number of the central metal ion is indicated by a Roman numeral written at the end of the name. 

In chelation complexes (sometimes called inner complexes when uncharged) the central metal ion coordinates with a polyfunctional organic base to form a stable ring compound, e.g. copper(II) acetylacetonate or iron(III) cupferrate ... 

Classical complexes are identified [1112] as those species in which the central metal ion possesses a well-defined oxidation number and a set of ligands with a discrete electron population. Non-classical complexes , in contrast, involve highly covalent and/or multiple metal-ligand bonding resulting in indistinct oxidation numbers for both participants. 

A Roman numeral denotes the oxidation number of the central metal ion ... 

Many other shapes are possible for complexes. The simplest are linear, with coordination number 2. An example is dimethylmercury(O), Hg(CI l,)2 (4), which is a toxic compound formed by bacterial action on aqueous solutions of I Ig ions. Coordination numbers as high as 12 are found for members of the / block, but they are rare in the d block. One interesting type of d-mctal compound in which there are 10 links between the ligands and the central metal ion is ferrocene, dicyciopentadi-enyliron(O), [Fe(C5H5)2] (5). Ferrocene is an aptly named sandwich compound, with the two planar cyclopentadienyl ligands the bread and the metal atom the filling. The formal name for a sandwich compound is a metallocene. 

As an example, let s consider an octahedral d] complex, such as one containing a Ti3+ ion. In a free Ti3+ ion, all five 3d-orbitals have the same energy and the d-clec-tron is equally likely to occupy any one of them. However, when a Ti3+ is dissolved in water, six H20 molecules surround it and form a [Ti(H20)h 31 complex. The six point charges representing the ligands lie on opposite sides of the central metal ion along the x-, y-, and z-axes. From Fig. 16.25, we can see that three of the orbitals (dxy, d, and d,x) have their lobes directed between the point charges. These three d-orbitals are called f2g-orbitals. The other two d-orbitals (dz2 and dx, y2), have lobes... 

FIGURE 16.33 In a ligand-to-metal charge-transfer transition, an energetically excited electron migrates from a ligand to the central metal ion. This type of transition is responsible for the intense purple of the permanganate ion, MnCF,. ... 

The replacement of the O—H O bridges with BF2 of BPh2 may affect both the complex geometry [178] and the electron density at the central metal ion [184], providing the opportunity of adjusting the Co—C bond strength towards homolytic cleavage, which is currently accepted to be the first step of the reactions catalyzed by the vitamin B12 coenzyme [185]. 

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

These compounds contain a developed system of conjugated double bonds imparting distinct semiconductor properties on them. Metal ions of variable valency can serve as the central ion M cobalt, nickel, iron, manganese, copper, and so on. In such systems, electron transitions can occur in the conjugated system of the ligands and in the electronic system of the central metal ion. These transitions are the basis for their catalytic activity toward various reactions. 

The catalytic activity of the N4 complexes depends both on the nature of the central metal ion and on the nature of the ligand and aU substituents. It was found that the metal ion is the active site where the electrocatalytic process is accomplished. During its adsorption, an oxygen molecule forms a stable complex (adduct) with the... 

An abbreviated version of this series can be arranged mainly according to the nature of the ligand atom directly bonded to the central metal ion ... 

The most fundamental approach to assessing lability of complexes is by determination of the rate of isotopic exchange reactions. In the technetium-complex systems, no study of the exchange reaction on the central metal ion has been reported, but several reports have been published on isotopic exchange by ligand substitution. 

Hiratsuka et al102 used water-soluble tetrasulfonated Co and Ni phthalocyanines (M-TSP) as homogeneous catalysts for C02 reduction to formic acid at an amalgamated platinum electrode. The current-potential and capacitance-potential curves showed that the reduction potential of C02 was reduced by ca. 0.2 to 0.4 V at 1 mA/cm2 in Clark-Lubs buffer solutions in the presence of catalysts compared to catalyst-free solutions. The authors suggested that a two-step mechanism for C02 reduction in which a C02-M-TSP complex was formed at ca. —0.8 V versus SCE, the first reduction wave of M-TSP, and then the reduction of C02-M-TSP took place at ca. -1.2 V versus SCE, the second reduction wave. Recently, metal phthalocyanines deposited on carbon electrodes have been used127 for electroreduction of C02 in aqueous solutions. The catalytic activity of the catalysts depended on the central metal ions and the relative order Co2+ > Ni2+ Fe2+ = Cu2+ > Cr3+, Sn2+ was obtained. On electrolysis at a potential between -1.2 and -1.4V (versus SCE), formic acid was the product with a current efficiency of ca. 60% in solutions of pH greater than 5, while at lower pH... 

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