Electron-rich metal center

There is an interesting paradox in transition-metal chemistry which we have mentioned earlier - namely, that low and high oxidation state complexes both tend towards a covalency in the metal-ligand bonding. Low oxidation state complexes are stabilized by r-acceptor ligands which remove electron density from the electron rich metal center. High oxidation state complexes are stabilized by r-donor ligands which donate additional electron density towards the electron deficient metal centre. 

While Fe(- -3)- and Fe(+2)-ate complexes are formed by the coordination of four anionic ligands, more electron-rich metal centers tend to bind neutral or even... 

Uson, R. (1989) Electron-rich metal centers (Au, Pt) as sources of organometallic complexes with unusual features. Journal of Organometallic Chemistry, 372, 171-182. 

The Carbyne Carbon. Protic and Lewis acids can add to the carbyne carbon of complexes containing electron-rich metal centers ... 

The facility with which electrophilic halocarbene complexes undergo substitution reactions makes them extremely versatile synthetic intermediates, and this section summarizes these synthetic possibilities. Scheme 3 illustrates the usefulness of RuCl2(=CCl2)(CO)(PPh3)2. When the ligands are bound to electron-rich metal centers the electrophilicity is much reduced and interaction of the M=C function with some electrophiles can be observed. 

The resistance of metal carbonyls to addition across the CO bond may reflect the influence of the adjacent electron rich metal center, which can delocalize electron density onto the car-... 

In the reaction of group 13 element halides with metal carbonyl dianions, the analysis is more complex than observed for the reactions with metal monoanions. Upon addition of metal dianions to EX3 or REX3, either one or two halide ions may be eliminated. When only one halide ion is eliminated per added metal dianion, the complexes may still be viewed as E3+ derivatives (Equations (33)-(36)).19 This may be controlled to some extent by the stoichiometry of the reaction. Comparison of Equations (33)19 and (34)19 shows that the electron demand at the main group element can be satisfied by coordination either to an electron-rich metal center 26 or formation of a halide bridge 27. Ligand-stabilized forms may also be prepared in this fashion (Equation (36)).19... 

It did not prove possible to synthesize a substituent-free Ga complex with formula Cp (CO)2Fe Fe(CO)4 Ga (Scheme 13).43 Addition of bipy to 30 resulted in halide elimation, but the main group element in the product 31 was coordinated by the bipy ligand. Upon addition of dppe, however, substitution of the carbonyl ligands occurred instead along with halide ion elimination to produce the substituent-free Ga complex 32. It has a linear coordination environment (Fe-Ga-Fe angle = 176.01(4)°), and the Ga-Fe bond distances are much shorter than in those related adducts where donor ligands are also bound to the Ga atom.43 The authors attributed the non-observation of the carbonyl derivative to a need for an electron-rich metal center to stabilize the Fe-Ga bond via 7r-backdonation. 

Polypyridyl ligands are able to stabilize electron-rich metal centers as has been shown for the d -systems Ru and Os. This suggests this class of ligands also to be appropriate for rhenium(I) compounds, and this will be discussed more in detail in Section 5.3.2.7.2, and rhenium(II) complexes which represent intermediates in the syntheses of the Re species or are accessed by oxidation of Re. ... 

The configuration of rhenium(I) requires ligand systems which are able to accept electron density from the electron-rich metal center. Thus, frequently phosphines, nitrogen heterocycles, carbonyls, or isocyanides are encountered. Most of the octahedral products possess a high thermodynamic stability and kinetic inertness as is expected for 18-electron systems. 

For the formation of the 4-ethynylquinoline complexes a mechanism was proposed involving nucleophilic attack of the terminal carbon of the butatrienylidene ligand at the imine carbon, followed by C—C bond formation between the ortho carbon of the N-aryl group and C3 of the butatrienylidene ligand. Deprotonation finally affords 4-ethynylquinoline complexes (Scheme 3.27). Some preference was observed for quinoline formation with the more electron-rich metal centers, whereas... 

Another theoretical study also showed that the third pathway (bl +b3+b4), 1,3 hydrogen shift, through a hydrido-alkynyl intermediate could compete with the 1,2 hydrogen shift pathway (bl+b2) when the metal center is electron-rich enough [29, 30]. Indeed several hydrido-alkynyl intermediates have been detected or even isolated during the q -l-alkyne-to-vinylidene rearrangement on electron-rich metal centers, such as Co(I), Rh(I) and Ir(I) [73-78]. The ab initio M P2 calculations by Wakatsuki, Koga and their coworkers on the transformation of the model complex RhCl(PH3)2(HC=CH) to the vinylidene form RhCl(PH3)2(C=CH2) indicated that the transformation proceeded via the oxidative addition intermediate RhCl(PH3)2(H) (C CH) [30]. 

Since the first report of a complex involving a direct metal-to-Ceo bond, (Ph3P)2 Pt( 7 -C6o)/ numerous studies have established that the fullerene Ceo acts as a moderately electronegative alkene in coordinating to electron-rich metal centers. In many cases the Ceo ligand is subject to relatively facile displacement when the complex is in solution however, the zerovalent, octahedral complexes M(CO)3(dppe)(Cgo) [M = Mo, W dppe = l,2-bis(diphenylphosphino)ethane] display outstanding stability even under severe conditions. The overall time needed to prepare these complexes from commercially available M(CO)g is dramatically reduced by adopting a biphasic procedure for the synthesis of the precursor M(CO)4(dppe), which was first described for the preparation of Mo(CO)4 (dppe). Here details are presented for the biphasic synthesis of W(CO)4(dppe) and for its use in the preparation of W(CO)3(dppe)(Cgo)-... 

Various calculations have shown that fullerenes will form especially strong bonds to electron-rich metal centers. This has been attributed to (a) favorable relief of bond strain and (b) electron-withdrawing fullerene having MOs highly energetically suitable for interaction. For harder metal centers such as Ag+, calculations have shown that the interaction is much weaker (89, 90). 

Overall, fullerenes and especially Ceo show a chemical reactivity very similar to that of bulky electron-deficient alkenes. They readily react with many electron-rich metal centers to form stable or a complexes. With either bulky or less electron-rich centers, they show a reduced reactivity and form much less stable complexes. 

Coordination of an alkene to platinum(O) differs from complexation to platmum(Il) Zerovalent platinum is an electron-rich metal center, whereas platinum(II) is electron poor. A a consequence alkenes coordinated to platinum(0) became more electron rich than in their fret state, and therefore susceptible to electrophilic attack. For alkenes complexed to platinum(II) their primary mode of reactivity is by attack from an external nucleophile. 

Presumably the complex forms by electrophilic attack of the C02 carbon on the electron-rich metal center, followed by a similar electrophilic attack of the second C02 on the more basic oxygen of the coordinated C02, forming an oxygen-carbon bond. The metallocycle ring closing then completes the complex formation. Support for this mechanism comes from infrared spectra implicating a mono-C02 adduct that is observed when the starting metal complex reacts with less than two equivalents of C02. 

Protonation reactions may be simply H+ addition in which the E-M framework of a cluster complex remains essentially unaltered. Where simple protonation is observed, conventional acid-base chemistry is involved and the site of protonation is almost always the electron-rich metal center, though protonation can sometimes be observed at E. Likewise, addition of bases to metal cluster hydrides results in the deprotonation [e.q., Eq. (243)179] Sometimes even halide ions in nonaqueous media are strong enough to deprotonate the metal centers [Eqs. (244)-(246)95 203,373]. Os-... 

The alkyl halides are much more susceptible to a nucleophilic attack by an electron-rich metal center than are the neutral or protonated alcohols. The relative importance of the halides in this role follows the classical halide sequence, that is, RI > RBr > RC1. 

Oxidative addition reactions of nonpolar molecnles, snch as H2, show quite different characteristics from the oxidative addition reactions of polar molecules, such as Mel. For H2 addition, the rate is relatively insensitive to the nature of the metal center, although a stable dihydride is formed only for very electron-rich metal centers. For frani -Ir(CO)(X)(PPh3)2, the rate depends on the X group (X = I > Br > Cl).i The very small deuterium isotope effect = 1-22) and... 

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