Metals in Normal Oxidation States

We arbitrarily define the majority of metal ions in oxidation states II and III as being in normal oxidation states, but we do not adhere rigorously to this classification. 

An infrared spectroscopic investigation suggests the phenanthroline molecules to be coordinated in M(phen)4(C104)2 (M = Ca, Sr, or Ba) (626). Treatment of calcium metal in liquid ammonia with the ligands at —70°C produces the solids Ca2(bipy)(NH3)i.29 (violet) and Ca(phen)(NH3)o,74 (purple) which become hot and turn yellow in air (699). They are probably best regarded as containing the ligand anions. 

It is said that alkyl- and arylmercurials do not form bipyridyl and phenanthroline complexes (151) however, perfluoroalkylmercurials are known to do so (133, 151, 172). A convenient preparation of such compounds has been described (170). Sutton (668) has reported the preparations of the compounds HgX2L, HgYL, and HgL (X = halogen, Y = oxalato or sulfato). 

The anhydrous tris-chelate complexes of aluminum perchlorate are stable beyond 300°C (669). Trialkylaluminums form colored complexes with bipyridyl and phenanthroline (109) which are five-coordinate monomers (681). 

Simple preparations of InLs(C104)3 and In(terpy)2(C104)3 are known (140, 669). All three are normal six-coordinate complexes of indium(III). The compounds In(bipy)3X3 (X = 01, Br, or I) have also been obtained 

Few complexes of gallium are known (560) although GaLsXg [X = CP, Br , or I (137), X = C104 (669)] have been reported. The reaction of bipyridyl with GaXg affords [Ga(bipy)2] [GaX4] (X = Cl or Br) (7). 

---

Bipyridine is a strong field ligand that forms relatively stable complexes, with the inherent M—N bond strength enhanced by the chelate effect. These factors favor the formation of 4-coordinate bis and 6-coordinate tris complexes. The tris complexes of the first row transition metals in normal oxidation states (+2 or +3) are best prepared by the reaction of a suitable metal salt with an excess of bpy in water, methanol, or other organic solvent. The solid complexes can be obtained by crystallization or by the precipitation of the perchlorate, hexafluorophosphate, tetrafluoroborate, or other salts. Because bpy is a strong field ligand, the lower oxidation states tend to be favored, and reduction of M(III) complexes can occur in these preparations. The M(III) complexes are usually readily obtained by the chemical, aerobic, or electrochemical oxidation of the M(II) species. 

Some simple alkyl and aryl compounds of metals in normal oxidation states can be considered as honorary coordination compounds. 

Reduction of Salts of Metals in Normal Oxidation States. In many cases the formation of reactive fragments can be achieved directly from salts of the metals in their normal oxidation states. In these processes the following two steps should be distinguished ... 

Some general comments on the solid-state chemistry ( From a molecular view on solids to molecules in solids ) have been reported by Simon (1995) emphasis was especially placed on the structural chemistry of metal-rich compounds formed by the metals in groups 1 to 6 and it was underlined that it is largely based on discrete and condensed clusters. In the chemistry of metals in low oxidation states, the residual valence electrons can be used for metal—metal bonding. Metal-rich compounds lie between normal valence compounds and the elemental metals themselves, with respect to their compositions, and often also with respect to their structures fragments of usual metal structures (close-packed, b.c.c., etc.) are often component units in the structures of metal-rich compounds. 

Bipyridyl (continued) as ligand, 12 135-1% catalysis, 12 157-159 electron-transfer reactions, 12 153-157 formation, dissociation, and racemization of complexes, 12 149-152 kinetic studies, 12 149-159 metal complexes with, in normal oxidation states, 12 175-189 nonmetal complexes with, 12 173-175 oxidation-reduction potentials, 12 144-147... 

For metal ions in normal oxidation states, the interaction of metal dn orbitals with the ligand n orbitals is significant, but not exceptional. However, these ligands can stabilize metal atoms in very low formal oxidation states and in such complexes there is extensive occupation of the ligand it orbitals, so that the compounds can often be best formulated as having radical anion ligands L. ... 

Transition metal cations in normal oxidation states Until the late 1970s, there had been little study of transition metal cations in HF because of the very low solubility of the di-, tri- and tetrafluorides of the d- and /-elements in HF. Use of counter-anions other than fluoride was precluded because the anions were protonated or solvolysed leading to formation of insoluble metal fluorides. The problem was overcome by treating the insoluble fluoride (MF ) in HF with an appropriate Lewis acid (AFm) ... 

Rven CO. which normally has little affinity for metals in high oxidation states. ha.s been found in WCU(PMePh2)i(C0)(0). Su. F.-M. Cooper. C. Geib. S. J. Rheingold. A. L. Mayer. J. M. J. A/ii. Chem. Sov. 1986, 0H, 3545-3547. Alkyl groups may also stabilize high oxidation states, c.g.. 

There is one report149) that claims that the addition of sulfide to oxidized cytochrome oxidase produces an EPR signal at g of 2.05 and 2.19 due to the previously undetectable Cu(II) (in addition to the normally observed Cu(II) peaks). That occurs immediately upon mixing, before the known reduction of the oxidase by sulfide can occur (1 min or less) and is claimed due to sulfide binding to the heme Fe(lII) which was previously coupled to the Cu(II). That report by Seiter, Angelos and Perreault149) is extremely significant and deserves detailed scrutiny and confirmation. It represents the only occasion of the simultaneous observation of the EPR spectrum of all four metals in the oxidized state. 

Complex oxides are normally found when a nonmetal is present, with oxoanions such as nitrate NO.v, carbonate -0 a > phosphate or sulfate SOj, but are also sometimes formed by metals in high oxidation states (e g. permanganate MnO jn KMnOA When a compound contains two... 

The methods of preparation are varied. Complexes involving transition-metal ions in normal oxidation states can usually be obtained by conventional reactions and then reduced with a variety of reagents such as Na/Hg, Mg or BH. The most general method employs Li2bipy ... 

The chemistry of the compounds of the early transition metals in low oxidation states is full of examples of the occurrence of metal - metal bonds. These compounds show unusual compositions in terms of the traditional valence rules metal - rich compounds contain more metal atoms than one expects for a normal valence compound (example Nbelii instead of Nbis). The transition metal elements on the left In the periodic system, and of those mainly the 4d and 5d elements, are capable of using the excess valence electrons, not needed to complete the octets of the anions, to fbnn metal -metal bonds. 

Oxidation State. Homogeneous catalysis normally involves the metal in changes of oxidation state. It can therefore speed up reactions if the catalyst already contains the metal in an oxidation state involved in the catalytic cycle. However, these oxidation states are often lower than those of readily available compounds. Thus a reduction step is generally involved in the preparation of homogeneous catalysts. An example of this is the use of platinum catalysts for hydrosilylation. 

Oxidative addition normally is favored for metals in low oxidation states. However, it has become increasingly clear that C—H activation also can be observed with M(II) and M(III) species. Probably the first observation of this was the report by Shilov and co-workers that substitutionally inert aqueous Pt(II) activates alkanes to form alcohols. However, this case was not clearly recognized because of mechanistic uncertainties. Subsequent to much of the detailed work with low oxidation states, examples of oxidative additions to Ir(III) and Rh(III) have been documented. 

The 18 electron-rule normally does not apply to transition metal complexes with 7i-donor ligands and metal atoms in normal oxidation states. In this case the metal orbitals with 7r-symmetry lie at relatively high energy and they stay empty or are only partially filled. In this way the formation of paramagnetic species can be possible. 

The limiting process for the stability of polycations is the disproportion reaction to the metal and compounds in normal oxidation states, for instance,... 

The STEM Is Ideally suited for the characterization of these materials, because one Is normally measuring high atomic number elements In low atomic number metal oxide matrices, thus facilitating favorable contrast effects for observation of dispersed metal crystallites due to diffraction and elastic scattering of electrons as a function of Z number. The ability to observe and measure areas 2 nm In size In real time makes analysis of many metal particles relatively rapid and convenient. As with all techniques, limitations are encountered. Information such as metal surface areas, oxidation states of elements, chemical reactivity, etc., are often desired. Consequently, additional Input from other characterization techniques should be sought to complement the STEM data. 

A detailed study of the mechanism of the insertion reaction of monomer between the metal-carbon bond requires quantitative information on the kinetics of the process. For this information to be meaningful, studies should be carried out on a homogeneous system. Whereas olefins and compounds such as Zr(benzyl)4 and Cr(2-Me-allyl)3, etc. are very soluble in hydrocarbon solvents, the polymers formed are crystalline and therefore insoluble below the melting temperature of the polyolefine formed. It is therefore not possible to use olefins for kinetic studies. Two completely homogeneous systems have been identified that can be used to study the polymerization quantitatively. These are the polymerization of styrene by Zr(benzyl)4 in toluene (16, 25) and the polymerization of methyl methacrylate by Cr(allyl)3 and Cr(2-Me-allyl)3 (12)- The latter system is unusual since esters normally react with transition metal allyl compounds (10) but a-methyl esters such as methyl methacrylate do not (p. 270) and the only product of reaction is polymethylmethacrylate. Also it has been shown with both systems that polymerization occurs without a change in the oxidation state of the metal. 

---