Alkylidynes, transition metals

Two commonly used synthetic methodologies for the synthesis of transition metal complexes with substituted cyclopentadienyl ligands are important. One is based on the functionalization at the ring periphery of Cp or Cp metal complexes and the other consists of the classical reaction of a suitable substituted cyclopentadienyl anion equivalent and a transition metal halide or carbonyl complex. However, a third strategy of creating a specifically substituted cyclopentadienyl ligand from smaller carbon units such as alkylidynes and alkynes within the coordination sphere is emerging and will probably find wider application [22]. 

Figure 4.16 Optimized structures of transition-metal alkylidynes H MCH (M = W-Ir).

Main-group elements X such as monovalent F, divalent O, and trivalent N are expected to form families of transition-metal compounds MX (M—F fluorides, M=0 oxides, M=N nitrides) that are analogous to the corresponding p-block compounds. In this section we wish to compare the geometries and NBO descriptors of transition-metal halides, oxides, and nitrides briefly with the isovalent hydrocarbon species (that is, we compare fluorides with hydrides or alkyls, oxides with alkylidenes, and nitrides with alkylidynes). However, these substitutions also bring in other important electronic variations whose effects will now be considered. 

For a general review of metal-ligand multiple bonds, see W. A. Nugent and J. M. Mayer, Metal-Ligand Multiple Bonds The Chemistry of Transition Metal Complexes Containing Oxo, Nitrido, Imido, Alkylidene, or Alkylidyne Ligands (New York, Wiley, 1988). 

The chemistry of alkylidene and alkylidyne complexes of early transition metals was developed by Schrock and co-workers and these complexes turned out to be of crucial importance to alkene and alkyne metathesis. Initially their research focused on tantalum complexes of the type CpTaCEIE, which after a-elimination (Figure 16.6) led to alkylidene complexes Cp(R)Cl2Ta=CHR [11]. 

A. Mayr, Comments on the Chemistry of Low-Valent Alkylidyne Complexes of the Group 6 Transition Metals, Comments Inorg. Chem. 10, 227-266 (1990). 

Bond dissociation enthalpies (BDEs see Box l.l) for transition metal o-organyls appear at present to fall within the quite wide range of 70-330 kJ mol-1, i.e. a range comparable to those of main group alkyls. It should be noted, however, that transition metals also have the possibility of including a 7t-component when the carbon is sp2 (alkylidenes, alkenyls, acyls) or sp hybridized (alkylidynes, vinylidenes, alkynyls), thereby leading to larger BDEs. 

Several factors affect the nature of the products in a reaction between a transition metal cluster and an alkyne or alkene. In this section, the various synthetic routes to alkyne or alkene-substituted clusters will be presented, and these will be used to analyze the changes in reactivity of the cluster systems when one or more of the important reaction parameters is altered. In order to simplify the discussion, tri-, tetra-, and higher nuclearity clusters will be treated separately. Finally, in this section, there is a brief description of the chemistry of alkylidyne-substituted clusters since synthetic routes to alkyne-containing complexes may involve these species. 

Bocarsly and Mayr first reported the luminescence properties of transition metal alkylidyne see Alkylidyne) complexes [XW(CO)2(L2)(CPh)] (X = hahdes, L2 = TMEDA, (py)2 or dppe). The complexes exhibited broad and stmctureless emission bands at ca. 625-640mn, which was attributable to an excited state of MLCT (dxy(W) - 7r (W=C)) character. With an extension of the alkylidyne unit, the complexes [XW(CO)2 (L2) (C-C6H4-/ -(C=C-C6H4-/ -) -N=C)] (L2 = TMEDA, dppe n = 0, 1) containing an isocyanide moiety have been shown to possess rich luminescence properties. Perturbation of the absorption and emission properties of the complexes by coordination of the isocyanide moiety to metal-containing fragments such as/ac-ReCl(CO)3, cw-PdCb, trans-Vdh, and trans-Vth was also studied. 

Photochemistry of Alkyl, Alkylidene, and Alkylidyne Complexes of the Transition Metals... 

Transition metal alkyl (1), alkylidene (2), and alkylidyne (3) complexes... 

The generation and interconversion of hydrocarbon fragments on metal surfaces is an important aspect of transition metal catalysis. In an effort to model and understand these transformations, much attention has been focused on the synthesis and reactivity of organic species coordinated at polynuclear transition metal centers. Organodiruthenium complexes have provided a particularly rich area of study. The availability of a variety of organometallic derivatives of the bis(T) -cyclopentadienyl)diruthenium carbonyl system has allowed extensive examination of the reactivity of bridging alkylidene, alkylidyne, and ethenylidene ligands. 

Figure 6.4. Alkylidyne structure of (a) ethylene [24] and (b) other alkenes on transition metal surfaces [24] and in (c) organometallic clusters [25].

Boron atoms in transition metal clusters Denuding the boron atom of B-H interactions in transition metal boron clusters Transition metal boride clusters at the molecular level Recent advances in the chemistry of carborane metal complexes incorporating d and /block elements The interplay of alkylidyne and carbaborane ligands at metal centres. I. Synthesis of electronically unsaturated mixed-metal complexes. II. Proton-mediated reactions... 

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