Schrock-type complexes

Phosphinidenes differ from carbenes because of the additional lone pair. This lone pair enables interactions with, e.g., a transition metal group for increased stability, while maintaining carbene-hke behavior. These terminal /] -complexed phosphinidenes differ from the p2-> fi3-> and p4-complexes, which are not part of this survey. Phosphinidenes that are stabilized by a transition metal group also relate to carbene complexes. A distinction in Fischer and Schrock-type complexes has been advanced to distinguish phosphinidene complexes with nucleophilic properties from those that are electrophiHc [ 13 ]. In this survey we address this topic in more detail. 

Fig. 4. Schematic drawing of the potential energy surfaces for the CPMD simulations. PES of Sellmann-type complexes (left) PES of Schrock-type complex (right).

Summary of the CPMD Simulation Results for the Schrock-Type Complexes. 

Alkylidene carbonyl iridium complexes, reactions, 7, 275 Alkylidene compounds, NLO properties, 12, 121 Alkylidene-containing complexes, in molybdenum complexes, Schrock-type complexes, 5, 524 a-Alkylidene cyclic carbonyl compounds, isomerization,... 

Alkylidyne-metal complexes have traditionally been divided into two categories, according to the oxidation state of the metals, in a manner directly analogous to the classification of the very large number of known alkylidene-metal species (19a,b). Hence Fischer-type alkylidyne complexes involve metals in low oxidation states, while Schrock-type complexes generally involve more electropositive metals with higher oxidation states (13). However, the properties of some of the numerous carbyne-metal complexes that have been characterized since the early days have in many cases blurred the distinction between the two classes (12a). 

Another type of carbene complex is characterized by a high oxidation state of the central metal and an a-carbon atom that does not usually bear a hetero atom. It is called an alkylidene complex or Schrock-type complex, since R. R. Schrock first synthesized a tantalum complex of this type [14]. Formation of the tantalum carbon double bond is based on the a-elimination reaction of a neopentyl ligand as shown in eq.(2). 

The M—Ccarbene bonds in both Fischer- and Schrock-type complexes are longer than typical M—Cco(term) bonds, but shorter than typical M—C single bonds, e.g. in (OC)5Cr=C(OMe)Ph, Cr-Ccarbene = 204 pm and Cr—Cco = 188 pm. This implies some degree of (fi -/j)7r-char-acter as indicated by resonance structures 23.51 and 23.52. The TT-system can be extended to the heteroatom in the Fischer-type system as shown in diagram 23.53. 

Carbenes are defined as species containing divalent carbon [1], and they may display either electrophilic or nucleophilic reactivity depending on whether the two unshared electrons on the carbon center are unpaired (triplet carbene) or paired (singlet carbene). Metal-carbene complexes can be classified in a similar way based on their reactivity toward electrophiles and nucleophiles. The resonance forms shown in Fig. 4.1 define the limiting structures, and the formal charge on the carbene carbon indicates the preferred reactivity. Those that are nucleophilic at carbon are called Schrock-type complexes or alkylidenes, and they generally... 

The M-Ccarbene bonds in both Fischer- and Schrock-type complexes are longer than typical M—Cco(term) bonds, but... 

Owing to their physicochemical properties and reactivities, carbene complexes may conveniently be divided into (1) Fischer-type coordination compounds and (2) Schrock-type complexes. Fischer-type complexes encompass 18 compounds of... 

There are two types of transition metal carbene and carbyne complexes low-valent (so-called Fischer type)i" i" and high-valent (so-called Schrock type). The two classes of compounds are quite different in their chemical behavior. Such different chemical reactivity is sometimes rationalized on the basis that the metal-carbene and metal-carbyne bonds in Fischer-type complexes have donor-acceptor character, whereas the bonding in Schrock-type complexes is more typical for a normal multiple bond. 

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