Complexes of Metals in Low Oxidation States

The preparation of many complexes of bipyridyl-containing metals in low oxidation states have been achieved by Herzog and his co-workers (e.g., 367). These and other compounds of interest are cited in Table XVI together with magnetic moments measured at ambient temperature (the most widely determined property). Although most work has been carried out with bipyridyl, it is apparent that phenanthroline and terpyridyl will afford similar complexes. There is a general paucity of physical data for the compounds listed in Table XVI. The determination of magnetic susceptibilities as a function of temperature would be worthwhile in many cases and the two iron compounds are obvious candidates for a Mossbauer study. 

Li[V(bipy)3]-4THF- Diamagnetic (583) Co(phen)3C104 No data reported (512) 

Earlier preparative studies used conventional reducing agents, e.g., magnesium metal (332, 334), but the usual method now is to use the lithium salt of the bipyridyl anion, Li2(bipy) (diamagnetic). For example. 

Tetrahydrofuran (THF) is the most commonly used solvent, although other ethers may be used. The sodium salts of bipyridyl or benzophenone may also be used as the reducing agent. 

An early claim (332, 334) to have prepared V(bipy)g+ has been revised (431) the compound V(bipy)3l is considered to be a mixture of V(bipy)3 and V(bipy) 3I2. The reaction of V(bipy ) or of Cr(bipy) + with aluminum hydride gives the neutral tris-bipyridyl complexes, but with other dipositive metals Al(bipy)3 is the major product (357). Attempts to prepare bipyridyl and plienanthroline derivatives of hexacarbonylvanadium led to the disproportionation of vanadium(O) (376). 

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Owing to the extensive sharing of valence electrons over the metal and donor atoms in complexes of metals in low oxidation states, the zero oxidation state has little, if any, physical significance, and may be used for classifying purposes only. Outstanding examples are the complexes with nitric oxide which may be considered as NO+, NO- or NO, and consequently different oxidation states may be assigned to the same complex. 

Complexes of metals in low oxidation states are generally electron rich and can act as bases, e.g. towards protons. If the complex bears a negative charge and contains electron—releasing ligands such as trialkylphosphines, the basicity... 

Complexes of metals in low oxidation state induce rearrangements and isomeiizations of strained and unsaturated hydrocarbons which occur with C-C bond cleavage [68] (Scheme IV.35). 1,4-Pentadiene can be isomerized (with splitting C-C bonds) under the action of hydride cyclopentadienyl complexes of scandium [69a]. Hydrogenolysis of C-C bonds in saturated hydrocarbons has been described in the presence of the system H2-Re2(CO)io-AI2O3 [69b, ... 

Since silicon is an analog of carbon in the Periodic System, it is important to survey reactions of Si-H compounds with complexes of metals in low oxidation states. Similar to C-H compounds, derivatives containing Si-H bonds easily react with low-valent metal complexes via oxidative addition mechanism to afford hydridosilyl or silyl compounds. Typical examples of such reactions [70] are shown below. 

Kinetic studies of substitution in tetrahedral complexes of metals in low oxidation states, for instance nickel tetracarbonyl or tetrakistrialkyl-phosphitenickel(o), are dealt with in the organometallic Chapter. Here we shall deal with reactions of oxoanions, isomerisation of tetrahedral complexes to square-planar forms and vice versa, and with the addition of ligands to tetrahedral to give octahedral complexes. 

Different valence states are also a fairly widespread type of unit variability. By analogy with macromolecular complexes (Section II), it may be expected that homopolymerization and copolymerization of metal-containing monomers would prevent or retard redox processes involving participation of metal ions. Experimental data confirm the fact that a polymeric matrix stabilizes complexes of metals in low oxidation states (e.g., Pd" ). Moreover, the stability of the Cu+ state during polymerization (including thermal polymerization) of copper acrylate controls the use of this method for the preparation of coordination compounds of Cu". The polymeric framework plays a stabilizing role, whereas the metal ions that are localized on the surface layer are oxidized to Cu +. However, polymerization of monomers that contain metal ions in high oxidation states is often accompanied by their reduction Fe + ->Fe +, and Mo + ->Mo" (scheme 14). For example, polymerization of Cu " and Fe + acrylates may be accompanied by intramolecular chain termination. This may be attributed to the relatively low standard reduction potentials of these metal ions (7io(Cu + Cu+) = 0.15, o(Fe ->Fe ) = 0.77 V). 

The expressions referred to above are derived from the ligand-field model, which is inappropriate for the highly covalent complexes of metals in low oxidation states and can lead to incorrect results. Thus, MO approaches, tailored to the particular... 

Oxidative additions occur of complexes when metals in low oxidation states (0, 1 +) are used to add AB compounds with rupture of the A—B bond. The addition of the new ligands A and B can take place to one or in some cases two metal centers, e.g. ... 

Organic isocyanides (C=N-R) are versatile ligands in transition metal complex chemistry. As compared with their pseudo-isoelectronic cousin, C=0, they are stronger o-donors [1], As a result, isocyanides form more stable complexes with metals in relatively high oxidation states (e.g., +2 and +3) than CO. In contrast, they have a lower ir-accepting ability than CO and therefore form less stable complexes with metals in low oxidation states (e.g., -1 and -2). Nevertheless, they form a broad range of metal complexes, and various aspects of their syntheses, structures and bonding have been reviewed [1-7]. 

Alkylidyne-metal complexes have traditionally been divided into two categories, according to the oxidation state of the metals, in a manner directly analogous to the classification of the very large number of known alkylidene-metal species (19a,b). Hence Fischer-type alkylidyne complexes involve metals in low oxidation states, while Schrock-type complexes generally involve more electropositive metals with higher oxidation states (13). However, the properties of some of the numerous carbyne-metal complexes that have been characterized since the early days have in many cases blurred the distinction between the two classes (12a). 

Olefin ligands are less common in complexes of metals in higher oxidation states, with charges greater than +1, and ivith d electron coimts. Yet, olefin complexes of d metals are important intermediates in olefin polymerization. Because of the importance of these d olefin complexes, the direct observation of such complexes has been sought, and such olefin complexes have recently been identified, usually at low temperatures. Two examples of d olefin complexes generated from the intermolecular coordination of ethylene are shown in Figure 2.21. 

Hydrogen cyanide may undergo oxidative addition to low-valent complexes or it may be coordinated as a Lewis base in the case of complexes of metals in higher oxidation states ... 

The theories of bonding in coordination compounds [3] have evolved subsequent to Werner s coordination theory (1893). Werner introduced the concept of primary and secondary valency, explaining the formation of the coordination compounds. The 18-electron rule, stating that the stable complexes with low formal oxidation states of metal ions should have 18 bonding electrons around the metal ion, became an important beginning point toward the study of the stabihty of the complexes. The 18-electron rule is significant in modem coordination chemistry as it is also supported by the molecular orbital theory. However, a smaller number of complexes with metals in low oxidation states restrict its wide applicability. An important advance in the theories of bonding in coordination compounds was the introduction of... 

To summarize, complexes with metals in low oxidation states, and ligands that are good acceptors, often have electron counts of 18 and sometimes 16. Electron counts of 14 may be encountered in reactive species or when bulky ligands are present. Early transition metal... 

Metal derivatives of terminal alkynes, RC2H. Transition metals form complex acetylides (e.g. (M(C = CR) ]- ) often containing the metal in low oxidation states. 

The S02 molecule has unshared pairs of electrons on both the sulfur and oxygen atoms. As a result, it forms numerous complexes with transitions metals in which it is known to attach in several ways. These include bonding through the sulfur atom, through an oxygen atom, by both oxygen atoms, and various bridging schemes. In most cases, the complexes involve soft metals in low oxidation states. Another important reaction of sulfur dioxide is known as the insertion reaction, in which it is placed... 

When a metal atom donates electron density to a bound ligand, usually by means of Ji-back bonding, electrophilic substitution reactions may be promoted. This is observed then usually with metals in low oxidation states and is therefore prevalent with organometallic complexes - and less with those of the Werner-type, where the metals are usually in higher oxidation states. Nevertheless there have been detailed studies of electrophilic substitution in metal complexes of P-diketones, 8-hydroxyquinolines and porphyrins. Usually the detailed course of the reaction is unaffected. It is often slower in the metal complexes than in the free ligand but more rapid than in the protonated form. 

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