Metal-carbene complexes spectra

The value of -NMR and 13C-NMR spectroscopy in characterizing transition metal carbene complexes was noted in Section III,B,2. The carbene carbon resonance is invariably found at low field (200-400 ppm) in the 13C-NMR spectrum, while protons attached to Ca in 18-electron primary and secondary carbene complexes also resonate at low fields. NMR data for some Ru, Os, and Ir alkylidene complexes and related compounds are given in Table V. 

At the beginning of Section 10-3, we commented that metal-carbene complexes exhibit a spectrum of reactivities with nucleophiles and electrophiles, especially at Qarbene- Carbene complexes of mid-transition metals (Groups 7-9) without heteroatomic substituents at Ccarbene may show electrophilic behavior depending upon the nature of other ligands, oxidation state of the metal, and overall charge on the complex. From some observations listed below, we may be able to discern a pattern of reactivity.63... 

It is interesting that the Ru alkylidenes that Grubbs discovered are both alkylidenes and complexes with the metal in a relatively low oxidation state (+2).35 As Grubbs has mentioned in his writings and conversations, there is a whole spectrum of reactivity in metal-carbene complexes, and it is difficult to classify the reactivity of many of these as being either Fischer type or Schrock type.36... 

The use of a metal carbene complex in benzannulations has become one of the most valuable synthetic applications of these organometallic reagents . Because of its applicability to a broad spectrum of substituents, its regioselectivity and its mild experimental conditions, benzannulation has been employed as a key step in the synthesis of a series of natural compounds . 

Non-heteroatom-substituted carbene complexes cover a broad spectrum of different reactivities, largely dependent on the electronic properties of the metal. In Chapter 1 the division of carbene complexes into Fischer-type and Schrock-type carbenes was discussed. This way of grouping carbene complexes, although difficult to apply... 

The only direct evidence for the presence of metal-carbene-olefin intermediates in catalytic metathesis systems comes from a study of the interaction of the tungsten cyclopentylidene complex 27 with cycloalkenes such as cycloheptene 28 in CD2CI2. When these are mixed at —96 °C and the temperature raised to between —53 and —28 °C, no polymerization occurs but the 13C NMR spectrum contains additional resonances which may be assigned to the metal-carbene-olefin complex 29. The line intensities show that the equilibrium 7 moves to the right as the temperature is lowered120. 

The vibrational v(CSe) bands occur in the 1060-1140 cmrange and are very intense. The free CSe molecule has a v(CSe) frequency of 1036 cmwhich has been obtained by extrapolation from the electronic spectrum of CSe. The C NMR chemical shifts for metal thiocarbonyl complexes are large. The C NMR signals for CS groups occur in the range 310-340 ppm C NMR signals for carbene complexes also occur in the same range. 

Another large family of d complexes with MLCT absorptions which, however, appear in the UV region comprises cyano complexes [1] (e.g. [M(CN)6]4- with M = Fe, Ru and Os and metal carbonyls [1,4,43] e.g. M(C0)6 with M = Cr, Mo, W). These MLCT bands may obscure less intense bands of different origin such as LF absorption. A characteristic long-wavelength MLCT band was also identified in the abs. spectrum of the carbene complex Mn(C5H5)(CO)2(CPh2) (X iax = 380 nm. Fig. 5) [44]. 

Cyclopropenes are high-energy compounds therefore, they are extremely reactive molecules exhibiting a wide spectrum of unusual reactivities extending far beyond the simple reactions typical of olefins [56b, 141]. Rearrangement of cydopropenes using transition metals has been extensively studied as a method for generating transition metal vinyl carbene complexes (Scheme 2.87). 

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