Low-valent complexes

In this context, homogeneous catalysis has been increasingly used since it can offer valuable advantages, notably in terms of selectivity and efficiency. Indeed, low-valent complexes of transition metals like nickel, palladium, or cobalt can react with many functionalities, thus allowing numerous C,C-bond forming reactions. 

Nickel associated with 2,2 -bipyridine has also been studied and the low valent complexes obtained by electroreduction are very efficient catalysts in the dimerisation of aryl compounds [26]. In addition, they have been used in combination with the sacrificial anode process which allows the running of such... 

This reactivity pattern is certainly unexpected. Why should low-valent complexes react as electrophiles and highly oxidized complexes be nucleophilic Numerous calculations on model compounds have provided possible explanations for the observed chemical behavior of both Fischer-type [3-8] and Schrock-type [9-17] carbene complexes. In simplified terms, a rationalization of the reactivity of carbene complexes could be as follows. The reactivity of non-heteroatom-stabilized carbene complexes is mainly frontier-orbital-controlled. The energies of the HOMO and LUMO of carbene complexes, which are critical for the reactivity of a given complex, are determined by the amount of orbital overlap and by the energy-difference between the empty carbene 2p orbital and a d orbital (of suitable symmetry) of the group L M. 

Co complexes, Buchwald reported the Ti-catalyzed carbonylative coupling of enynes-the so-called Pauson-Khand-type reaction [28]-and realized the first such catalytic and enantioselective reaction using a chiral Ti complex [29]. Here, a variety of enynes were transformed into bicyclic cyclopentenones with good to high ee-values however, several steps were required to prepare the chiral Ti catalyst, while the low-valent complex proved to be so unstable that it had to be treated under oxygen-free conditions in a glove box. 

Processes following the mechanism described in Eq. (9) are the common mechanism of reaction for low valent complexes with ligand... 

Alkenes are known to form d - complexes with low valent transition metal ions (or atoms), thus stabilizing their low valent complexes (152). Complexes of this type are key intermediates in a variety of catalytic processes, e.g., hydrogenations, polymerizations,... 

A variation of this reaction involves the oxidative addition of iV-stannylketimines to low valent complexes (equation 91). 

The term oxidative cyclization is based on the fact that two-electron oxidation of the central metal occurs by the cyclization. The same reaction is sometimes called reductive cyclization . This term is based on alkene or alkyne bonds, because the alkene double bond in 13 is reduced to the alkane bond 14, and the alkyne 15 bond is reduced to the alkene bond 16 by the cyclization. Cyclizations of alkynes and alkenes catalyzed by transition metal complexes proceed by oxidative cyclization. In particular, low-valent complexes of early transition metals have a high tendency to obtain the highest possible oxidation state, and hence they react with alkynes and alkenes forming rather stable metallacycles by oxidative addition or oxidative cyclization. 

Low-valent complexes. Pons and Santelli1 have reviewed reductions with low-valent complexes of transition metals, particularly those of titanium and vanadium (301 references). 

The preparation of low-valent complexes such as [99mTc(CN-l )6]+ from water might still initiate research in the field of organometallic compounds for fife science. The paradigm that organometallic compounds are not compatible with conditions imposed by biological systems could clearly be repudiated with this compound. 

It is apparent from the examples in this section that the lability of the groups on silicon is greatly dependent on the catalyst. With conventional acid or base catalysts, SiO— is classed as a labile ligand, SiH as semi-labile, and Si—R (R = alkyl or aryl) as nonlabile (/). However, with low-valent complexes of the group VIII metals, SiH is the most labile ligand, and SiO— and Si—R appear to have comparable reactivities. Hence, these two sets of catalysts types are complementary in their capacity to redistribute ligands on silicon. 

Metal-vapour-deposition methods have begun to provide new low-valent complexes of uncommon stoichiometry. The preparation of gram quantities of paramagnetic (ji = 2.02 BM) [Co(bipy)2] has been reported and its reactions with Br2, tetracyanoethylene and tetracyanoquinodimethane studied.283 Electrochemically it behaves identically to [Co(bipy)3]2+ (equation 47).283... 

The majority of these contain CO ligands. The neutral homoleptic carbonyls have not been isolated, but anionic M(CO)6 as well as highly reduced M(CO) species are known. The original synthesis of the -1 species required elevated temperatures and high pressure but recently two simple, atmospheric pressure methods have been developed. They involve reduction of pentahalides in pyridine with Zn/Mg or in dimethoxyethane with sodium naphthalenide under an atmosphere of CO. These yellow salts contain discrete M(CO)6 anions. The facile syntheses of the octahedral hexacarbonyl anions allowed systematic exploration of the previously difficult to access area of low-valent complexes of Nb and Ta. Since M(CO)s are rather inert towards displacement of CO the substitution products of general formula M(CO)6- L have to be obtained by other routes, for example, by reduction of MX(CO)6- L compounds. The monosubstituted derivatives are conveniently prepared via the following method87 ... 

Pt(0), NVE 16/Pt(II), NVE 16). Oxidative additions generally occur most readily for low valent complexes, and for metals in the order 5d > Ad 3d. In addition to those that formally cleave C-X, H-X, and X-X (X = halide) bonds, oxidative addition reactions are also known where the metal is inserted into C-0, C-H, and some strained or activated C-C bonds. Another reaction which is effectively an oxidative addition is the formation of metallacycles from a low valent metal and an olefin (Equation 7). 

Although normally PF3 stabilizes low oxidation states of transition metals it has been possible in some cases to directly oxidize a low-valent complex with chlorine (method F). 

Nitrosyl complexes are rather numerous and [Re(NO)Xs] is known for all the halides (= X) addition of phen or bipy (= L-L) gives [Re(NO)X3(L-L)] or of pyridine, alcohols, DMSO, or MeCN (= solv) gives [Re(NO)X4(solv)] PPhs in EtOH followed by NaBH4 gives the nitrosyl hydride [ReH2(NO)(PPh3)3]. Other nitrosyl complexes include [Re(NO)Cl2(OMe)(PPh3)2] and [Tc(NO)Br4] and the low-valent complexes indicated above. 

In Section II,C, the oxidative-addition reaction of protonic acids to low-valent complexes was discussed. This reaction can lead to formation of either the hydro complex or to the bis complex of the conjugate base. Both of these reactions have been reported for the addition of silanes to low-valent complexes of the nickel triad (73, 74). It is to be expected that the nature of the product will depend on the strength of the acid HX and the trans influence of the conjugate base X. If the acid is strong and the conjugate base has a large trans influence, then it would be anticipated... 

Sensitization and quenching processes are well-known tools for obtaining information on the natures and behaviors of excited state organic molecules (211,212). These techniques have also been applied in photochemical studies of coordination compounds (204). However, only very recently have low valent complexes been used very much in these studies. Information on the techniques (204,211,212) and complications (204,213) of photosensitization and quenching processes is available elsewhere. 

Incorporation of pnictogen and chalcogen donors into vanadium compounds can be accomplished via oxidation of low-valent complexes. V(CO)g reacts with Ph2PPPh2 or PR2H (R = H, Me, Ph) to form compounds with formal V=V bonds ... 

Low-valent complexes of Mo and W undergo photochemistry, e.g., the Nj complexes M(N2)2(dppe)2 [M = Mo or W, dppe = l,2-bis(diphenylphosphino)ethane] react with alkyl bromides to form alkyldiazenido complexes, e.g. ... 

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