Ligands carbon monoxide

Chromium forms a white solid, hexacarhonyl, Cr(CO)j, with the chromium in formal oxidation state 0 the structure is octahedral, and if each CO molecule donates two electrons, the chromium attains the noble gas structure. Many complexes are known where one or more of the carbon monoxide ligands are replaced by other groups of ions, for example [CrfCOlsI] . 

With an atomic number of 28 nickel has the electron conflguration [Ar]4s 3c (ten valence electrons) The 18 electron rule is satisfied by adding to these ten the eight elec Irons from four carbon monoxide ligands A useful point to remember about the 18 electron rule when we discuss some reactions of transition metal complexes is that if the number is less than 18 the metal is considered coordinatively unsaturated and can accept additional ligands... 

Lithium 1,2,4-triazolate with [Rh2( j,-Ph2PCH2PPh2)(CO)2( j.-Cl)]PFj. gives the A-framed complex 177 (L=L = CO) (86IC4597). With one equivalent of terf-butyl isocyanide, substitution of one carbon monoxide ligand takes place to yield 177 (L = CO, L = r-BuNC), whereas two equivalents of rerr-butyl isocyanide lead to the product of complete substitution, 177 (L = L = r-BuNC). The starting complex (L = L = CO) oxidatively adds molecular iodine to give the rhodium(II)-rhodium(II) cationic species 178. 

Aryl- and alkenylcarbene complexes are known to react with alkynes through a [3C+2S+1C0] cycloaddition reaction to produce benzannulated compounds. This reaction, known as the Dotz reaction , is widely reviewed in Chap. Chromium-Templated Benzannulation Reactions , p. 123 of this book. However, simple alkyl-substituted carbene complexes react with excess of an alkyne (or with diynes) to produce a different benzannulated product which incorporates in its structure two molecules of the alkyne, a carbon monoxide ligand and the carbene carbon [128]. As referred to before, this [2S+2SH-1C+1C0] cycloaddition reaction can be carried out with diyne derivatives, showing these reactions give better yields than the corresponding intermolecular version (Scheme 80). 

The Pauson-Khand reaction (PKR) [96] consists of the synthesis of cyclopen-tenones by reaction of an alkene with a dicobalthexacarbonyl complexed alkyne (Scheme 57) and has recently emerged as one of the methods of choice for the obtainment of five-membered carbocyclic rings [97]. Its unique atom connectivity, which involves the two unsaturated carbons of the reagents and the carbon atom of a carbon monoxide ligand of cobalt usually in a regioselective manner (Scheme 57), has brought to refer to PKR as a [2 -I- 2 + 1] cycloaddition. 

For small-molecule, metal-carbon monoxide complexes, the carbon monoxide ligand is almost always in a linear conformation and perpendicular to the metal. If one assumed bonding of CO to Hb or Mb in its normal linear, perpendicular mode, steric conflicts as illustrated in Figure 4.20 would occur and thus one might predict... 

Kondo and Watanabe developed allylations of various types of aldehydes and oximes by using nucleophilic (7r-allyl)ruthenium(ll) complexes of type 154 bearing carbon monoxide ligands (Equation (29)).345 These 73-allyl-ruthenium complexes 154 are ambiphilic reagents and the presence of the carbon monoxide ligands proved to be essential to achieve catalytic allylation reactions. Interestingly, these transformations occur with complete regioselectivity only the more substituted allylic terminus adds to the aldehyde. 

There are two principal modes of coordination of the carbon monoxide ligand in transition metal carbonyls terminal coordination, to a single metal atom, and bridging coordination, to two or more metal atoms. The Ols spectrum of Co4(CO)12, shown in Fig. 5, can be readily deconvoluted into two peaks corresponding to these two types of carbonyl groups109. This spectrum is useful for determining the relative chemical shifts for the two types because... 

Generally phenol formation is the major reaction path however, relatively minor modifications to the structure of the carbene complex, the alkyne, or the reaction conditions can dramatically alter the outcome of the reaction [7]. Depending on reaction conditions and starting reactants roughly a dozen different products have been so far isolated, in addition to phenol derivatives [7-12], In particular, there is an important difference between the products of alkyne insertion into amino or alkoxycarbene complexes. The electron richer aminocarbene complexes give indanones 8 as the major product due to failure to incorporate a carbon monoxide ligand from the metal, while the latter tend to favor phenol products 7 (see Figure 2). 

In conclusion, the presented dinuclear iron structure is the first example of a bio-mimetic iron compound, which can be regarded as a first generation model for the class of [Fe]-only hydrogenases. The complex incorporates both relevant carbon monoxide ligands, as well as three bridging thiolato ligands, which could be possibly present in the active site of these enzymes. 

Metal-carbon monoxide ligand bond lengths in these same compounds have already been provided in Table 5-11. 

Substitution of a carbon monoxide ligand of complexes, such as 1, by the more electron-donating triphenylphosphane group (see Section 1.1.1.3.4.1.3.) provides chiral monophos-phane complexes, such as 3. Monophosphane complexes in general lack sufficient electrophilic-ity to react with amines or thiols, but react readily with amine anions at the /J-position, producing enolate anions such as 4, which may be quenched stereoselectively at the a-carbon by electrophiles46 (see Section 1.1.1.3.4.1.3.). The conformational and stereochemical issues involved are essentially identical to those already discussed in this section for the 1,4-additions of carbon nucleophiles. 

Treatment of the /J-hydroxy complex 15 with two equivalents of strong base followed by alkylation produces a mixture of the diastereomers 20 and 21 with an anomalously low d.r.27. The low degree of diastereofacial discrimination has been rationalized by invoking the formation of both rotamers of the initially formed alkoxide, 16 and 17. Rotamer 16 undergoes a-proton abstraction by a second equivalent of base to form the chelated dianionic Tf-enolate 18 which upon alkylation affords the usual diastereomer 20. Rotamer 17 is thought to rapidly transform to a metallo-lactone species by intramolecular attack of the alkoxide upon the proximate carbon monoxide ligand, which must occur faster than conversion to the less sterically encumbered conformer 16. Subsequent deprotonation to generate dianion 19, which is constrained to exist as the unusual Z-enolate, followed by alkylation provides the other diastereomer 21, which is formed in an amount nearly equal to 20. 

Alkylation of enolates, such as 4, produces products that are consistent with the preferred approach of the electrophile from either the least hindered face of an T -cnolate of conformation C or the least hindered face of a Z-enolate of conformation D88. Steric factors influencing approach of the electrophile appear to be similar in both of these models since the steric bulk of the hydridotris(3,5-dimethyl-l-pyrazolyl)borate ligand and the phosphite are both considerable any stereoelectronic and dipolar factors due to interaction of the enolate ligand with the carbon monoxide ligand would likely be similar for both geometries. The is-enolate geometry C appears to benefit from reduced steric interactions between the R substituent and the metal ligands. 

Stoichiometric metal carbene reagents undergo reactions that typify those of their catalytic counterparts, but it is often the appendages and carbon monoxide ligands of the metal that provide the synthetic versatility of these reagents. 

There is often a striking similarity between olefins and carbon monoxide as ligands, and one of the most common ways of preparing olefin complexes is by replacement of one or more carbon monoxide ligands in a metal carbonyl by olefins. Both olefin and carbonyl complexes frequently obey the simple E.A.N. rule, each CO ligand and C C bond contributing two ir-electrons to the metal atom, to enable it to attain the electronic configuration of the next inert gas in the Periodic Table. 

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