Carbyne complexes charge

Charge. The small amount of charge distribution data for carbyne complexes (based on Mulliken population analyses) indicates that the metal-carbon bond is generally polarized Ms+—C5- and that the carbyne carbon is always more negative than adjacent carbonyl carbons (28,30). These conclusions are directly analogous to those derived for carbene complexes. 

The reaction is thought to proceed with the dissociation of CT followed by release of the extra charge of the mthenium complex by dissociating a proton from the alkyhdene hgand. Such an exchange in itself does not lead to the decomposition of the alkyhdene complex. Nevertheless, both the formation of the charged species, both the intermediate existence of the carbyne complex (Scheme 9.5) may open new ways to the deterioration of the ROMP catalysts. 

These results suggest the presence of two competing pathways to products, which depend upon the location of protonation at the M=C carbyne bond. Charged controlled protonation at the carbyne carbon followed by nucleophilic attack of the CT leads to the butadiene complex. Frontier control of protonation results in attack at the metal center, leading ultimately to the hydride complex164. This has been verified by reaction of the... 

These play an enormous role in organometallic chemistry and will be mentioned frequently. The nature of the M to C bonding is very much dependent on the nature of M and R. At one extreme there are compounds in which M is in a high valence state and the R group(s) not of jr-donor character. In these cases, the bonds are comparable to those just discussed for M=NR and M=N. For these types of compounds, the terms alkylidene (M=CR2) and alkylidyne (M=CR) have been favored. On the other hand, when the metal is in a low valence state and the substituents on carbon are n donors, the M—C bonds are not full double or triple bonds and the systems are rendered stable by the migration of charge from the substituents (such as OR or NR) onto the carbon atom, as shown in resonance terms in (16-VI). For these compounds it is customary to use the terms carbene complex and carbyne complex. The chemistry is qualitatively different for the two classes. 

As with carbene complexes, metal carbynes display a range of reactivity with electrophiles and nucleophiles. Molecular orbital calculations show that even cationic Fischer carbyne complexes are polarized as M, +=C A neutral Fischer- and Schrock carbyne complexes have an even greater negative charge on Ccarbyne.93 If all reactions between carbyne complexes and other species were charge-controlled, we would predict that nucleophiles would always attack at the metal and electrophiles at Ccarbyne. As we should expect by now, the picture is more complicated in practice. 

Fig. 13.14. Schematic representation of the dominant orbital interactions in (a) Fischer-type carbene complexes (b) Schrock-type carbene complexes (c) Fischer-type carbyne complexes using charged closed-shell fragments (d) Fischer-type carbyne complexes using neutral open shell (doublet) fragments (e) Schrock-type carbyne complexes using neutral open-shell (quartet) fragments.

Examples of the electrophilic modification of a-carbons also include electrophilic attack at carbyne complexes. Calculations suggest that carbyne carbons bear a significant negative charge, but reports of electrophilic attack upon these carbons are rare. In Equation 12.40, protonation of CUMejPl WsCR gives a distorted alkylidene complex the C-H bond interacts with the coordinatively unsaturated metal in an "agostic" fashion. 

Fenske s theoretical studies on cyclopentadienyl-substituted carbyne complexes indicate a remarkable charge on the carbyne carbon atom [25]. This existence of a nucleophilic carbyne carbon becomes evident in the protonation of different carbyne complexes of molybdenum [26], tungsten [27-29]) and osmium [30]. 

Carbynes, (RCj), are 2e ligands and their carbon atoms have sp hybridization (two p orbitals are not hybridized). Therefore, in carbyne complexes L M —CR, two molecular orbitals d -p are formed. Thus, formally, the metal-carbon bond is triple. Calculations of electronic structures of [(OC)5Cr = CH] and [(OC)4ClCr=CH] show that the carbyne carbon charge is negative. Because in cationic carbyne complexes the carbon atom reacts with nucleophiles, this carbon s reactivity is not controlled by the charge, contrary to these calculations. The contradictions between calculations and experiments may be explained " by means of the frontier orbital theory (reactions controlled by frontier orbitals). Nucleophiles react with the atom of a molecule on which the LUMO is located, while electrophiles react with the atom of a molecule on which the HOMO is mainly localized. In the case of [(OC)5Cr = CH], the maximal coefficient in LUMO is at the carbyne carbon atom, and therefore it reacts with the nucleophile despite the fact that it is negative. 

Nonparametrized Fenske-Hall molecular orbital calculations on the carbyne complexes fra s-[CrX(CO)4(CR)] (X = Cl, Br, or I R = Me, Ph, or NEt2) support an earlier contention that nucleophilic additions to carbyne ligands are frontier orbital controlled rather than charge controlled. Thus, the only such complex to undergo nucleophilic addition to its carbyne carbon atom, namely, trans-[CrBr(CO)4(CC6H4Me-p)], is the only one whose LUMO corresponds to a Cr-carbyne ir-antibond. 

A few simple examples should make this electron counting rule clearer. In Cr(CO)6, 16.17 [I I], the CO groups donate two a electrons each for a total of 12 electrons. The charge on the molecule and each CO is zero, so Cr is in the zero oxidation state, that is, it is a d complex (see 16.16). The total number of electrons associated with Cr is 6 + 12 = 18. In The Fischer carbyne complex [12], 16.18, the carbyne ligand is taken to be a cationic 2-electron cr donor (see 16.8) and Br is counted as an anionic two-electron donor (16.9). Together with the four phosphine... 

In organic reactions there is abundant evidence for transient carbonium ions (R3C+), carbanions (R3C ), and carbenes ( CR2). Some stable carbonium ions like Ph3C+ and carbanions like C(CN)3 can be isolated as well as radicals like Ph3C In most of these cases the charge on the electron must be delocalized over the entire system for stability. Transition metal complexes with carbene or carbyne ligands, L M=CR2 and L M=CR, are discussed in Chapters 16 and 21. 

CH2BU or p-MeCjH4]. The reaction is considered to be charge controlled and to proceed via electrophilic attack by Sg on the carbyne carbon (178). An additional complex (267, M = W, E = S, L = CO, R = Me) can be obtained from [W=CMe(Cp)(CO)2] and cyclohexenesulfide (179). With the osmium complex 268 (R = /7-tolyl), however, reaction with sulfur does not proceed beyond the t/ -thioacyl complex (269, E = S or Se). Seleno-and telluroacyl complexes result from analogous reactions (180). 

This complex may be considered a dicarbyne species if the carbyne-type ligands are regarded as having a formal (1-) charge, i.e., MeHNC =5 W = CNHMe2. Alternatively, it may be considered as a dicarbene complex. 

---