7r-Acceptors

Osmium(II) forms no hexaaquo complex and [Os(NH3)g] +, which may possibly be present in potassium/liquid NH3 solutions, is also unstable. [Os(NH3)5N2] and other dinitrogen complexes are known but only ligands with good 7r-acceptor properties, such as CN, bipy, phen, phosphines and arsines, really stabilize Os , and these form complexes similar to their Ru analogues. 

Nevertheless, Cu can be stabilized either in compounds of very low solubility or by complexing with ligands having 7r-acceptor character. Its solutions in MeCN are stable and electrochemical oxidation of the metal in this solvent provides a convenient preparative route. The usual stereochemistry is tetrahedral as in... 

A few thionitrosyl complexes have been synthesized. MO calculations suggest that NS is a superior <7-donor and 7r-acceptor to NO [128], Syntheses include... 

C2F4 displaces one ethene to give Rh(C2H4)(C2F4)(acac), as does hexa-fluorodewarbenzene, whereas other alkenes (e.g. propene, styrene, vinyl chloride) displace both ethenes. Comparison of the structures of two complexes (Figure 2.28) shows that the Rh-C bonds are shorter to tetra-fluoroethene, because C2F4 is a better 7r-acceptor, with concomitant strengthening of the Rh-C bond. 

IR data [131] shows a trend to increasing i/(C-0) as the substituents on the phosphine became more electron withdrawing (Table 2.11) so that as the cr-donor power of the phosphine decreases and the 7r-acceptor power increases, the electron density at Ir decreases and electrons are removed from the "-orbital of CO [132]. 

There is a similar trend relating t/(C—O) to the 7r-acceptor strength of X (Figure 2.74). 

Additionally, MO calculations indicate the lowest energy orientation occurs with the three strongest trans-influence ligands (two hydrides and a PPh3) in a facial configuration. Calculations on compounds IrX(CO)(PR3)2 indicates that weak donors X and strong 7r-acceptors PR3 favour addition in the XIrCO plane [134, 135],... 

The reason for the greater stability of M(PR3)n over M(CO)4 must lie in the difference in donor characteristics of the two kinds of ligand. CO is a poor (7-donor but a strong 7r-acceptor, while tertiary phosphines are much better [Pg.196]

Complexes of bulky substituted phenanthrolines [Pt(N-N)LX2] (L, X both monodentate N-N, e.g. 2,9-dimethyl- 1,10-phenanthroline) can be 5-coordinate tbp when a good 7r-acceptor (e.g. C2H4) is present or 4-coordinate with monodentate phenanthrolines. Hartree-Fock calculations indicate that the 7r-acceptors reduce the electron density at platinum so that the metal can accept charge from another donor. Species of this kind may be involved in alkene hydrogenation [138]. 

One or two ligands such as CO and CN that have high observed trans-effects (and therefore are out of place in the above series) do possess empty orbitals that can act as 7r-acceptors to remove electron density from the metal ion, making the region trans to the ligand electron deficient and able to be attacked by the nucleophile in the transition state (Figure 3.84). 

Figure 3.84 Effect of a 7r-bonding ligand in acting as a 7r-acceptor. (Reproduced with permission from S.A. Cotton and F.A. Hart, The Heavy Transition Elements, published by Macmillan Press...

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

One additional point should be discussed here, concerning the substantial emphasis that has been placed on the differences between alkyl and aryl isocyanides. It has been suggested, primarily on the basis of infrared evidence, that aryl isocyanides are better 7r-acceptors than alkyl isocyanides (90). Qualitatively this difference is easily rationalized. One can see that delocalization of charge into 7r -orbitals on an aryl ring in aryl isocyanide-metal complexes should be possible, whereas no such possibility exists for alkyl isQcyanide-metal complexes this means that aryl isocyanides should be better ir-acceptors. Of course, the simple qualitative model gives one no measure of the relative importance of this effect. 

It is also appropriate to observe the substantial difference in ease of oxidation between phenyl and methyl isocyanide complexes, which has obvious chemical implication. This may perhaps be rationalized on the basis that phenyl isocyanide is a better 7r-acceptor than methyl isocyanide. This conclusion had previously been reached by infrared studies (see Section II). 

The monovalent Co chemistry of amines is sparse. No structurally characterized example of low-valent Co complexed exclusively to amines is known. At low potentials and in non-aqueous solutions, Co1 amines have been identified electrochemically, but usually in the presence of co-ligands that stabilize the reduced complex. At low potential, the putative monovalent [Co(cyclam)]+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) in NaOH solution catalyzes the reduction of both nitrate and nitrite to give mixtures of hydroxylamine and ammonia.100 Mixed N-donor systems bearing 7r-acceptor imine ligands in addition to amines are well known, but these examples are discussed separately in Section 6.1.2.1.3. 

The complex [(cp )IrCl(2,2 -bpy-4,4 -COO(CH2)3-pyrrole)]C104 has a typical three-legged piano-stool structure, with short Ir-cp distances due to the strong 7r-acceptor properties of the bpy ligands.540... 

N-heterocyclic compounds containing six-membered rings (pyridine and analogues) behave as excellent -acceptors and in turn they provide a rather soft site for metal ion coordination. The 7r-excessive five-membered pyrazole is a poorer 7r-acceptor and a better 7r-donor and it acts as relatively hard donor site. Inclusion of six- and five-membered N-heterocycles like pyridine and pyrazole in one ligand system leads to very attractive coordination chemistry with variations of the electronic properties.555 The insertion of a spacer (e.g., methylene groups) between two heterocycles, which breaks any electronic communication, makes the coordination properties even more manifold. 

Since most Ni1 species with simple N-donor ligands are prone to disproportionation into Ni° and Ni11, relatively few Ni1 complexes with nonmacrocyclic N-donor ligands have been reported. Formation of Ni1 species is in most cases proposed on the basis of electrochemical data, although ligand-centered redox processes have to be considered. The ligands usually contain imine donor atoms or aromatic N-heterocycles, which because of their 7r-acceptor ability favor stabilization of lower oxidation states. 

Nitrogen heterocycles have a strong affinity for platinum(II), and complexes of these ligands, particularly pyridine and related diimines such as bipy, are ubiquitous.189,190 The favorable electronic (strong cr-donor/weak 7r-acceptor) and steric properties of such ligands lead to the... 

Most gold(i) isocyanide complexes are colorless, crystalline materials, stable in air at room temperature, and soluble in common organic solvents. The compounds have been generally well characterized by IR/Raman and NMR spectroscopy, and crystal structures have been determined for almost a 100 examples. From the spectroscopic and structural data, it has been concluded that gold(i) isocyanides are strong cr-donors and poor 7r-acceptors. 

Dialkylzinc compounds have a tendency to form colored, and sometimes paramagnetic, adducts with 7r-acceptor ligands. The potentially bridging nitrogen donor pyrazine (and 4,4 -bipyridine) formed dinuclear complexes, Scheme 31, while the chelating 2,2 - bipyridine formed mononuclear adducts with diisopropylzinc.78 In the bis(diisopropylzinc)-pyrazine adduct 33, the presence of an unpaired electron was detected, but the resolution of the ESR spectra was insufficient for an unambiguous structural assignment. 

Diorganoboryloxides, R2BO, bearing bulky organic substituents are quasi-alkoxide ligands for main group and transition metals. Because of the 7r-acceptor properties of boron, these anions are weaker 7r-donors than alkoxides, and this is reflected in their structural chemistry. Diethylzinc reacted with the borinic anhydride (9-BBN)20, Scheme 82, to afford the alkylzincboryloxide [EtZnO(9-BBN)]4 129, which also crystallizes in heterocubic form.187... 

The presence of a remote C=C double bond in the alkyl halide was critical saturated analogs underwent nickel-catalyzed halogen-zinc exchange.248 It has been suggested that the double bond coordinates to the nickel atom. As a 7r-acceptor, the C=C bond removes some electron density from the metal, thus facilitating the reductive coupling (Scheme 154).246... 

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