Alkene and alkyne complexes

The C==C bond of the alkene lengthens on binding. Tlie M—alkene a bond depletes the C=C ir bond by partial transfer of these electrons to the 

Structural studies are best for determining where any given alkene complex lies on the structural continuum between 53 and 5.4. The position of any 

The same factors that lead to lowering of v(CO) in metal carbonyls also lead to greater metalacyclopropane character in alkene complexes strong donor coligands, a net negative charge on the complex ion, and a particularly low oxidation state for the metal. This means that Pd(II), Hg(II), Ag(I), and Cu(I) alkene complexes tend to be L-type, or Dewar-Chatt, in character, while those of Ni(0), Pd(0), and Pt(0), tend to be X2, or metalacyclopropane-like. 

Synthesis Alkene complexes are usually synthesized by one of the methods shown in Eqs. 5.1-5.7  

Reduction of a higher valent metal in the presence of an alkene  

CC02Et HCsCPh MeCsCC02Me PhCsCPh, In solid volatile [Cu(MeCsCMe) (CF3COCHCOCF3)] (19) the butyne ligand is parallel to the CF3COCHCOCF3 plane thermolysis of (19) provides Cu. 

The Organvtneudlic Chemistry of the Transition Metals, Fourth Edition, by Robert H. Crabtree Copyright 2005 John Wiley Sons, inc. 

Structural studies are best for determining where any given alkene complex lies on the structural continuum between 53 and 5.4. The position of any vinyl protons, or of the vinyl carbon.s in the H and C NMR, also shows a correlation with the structure. For example, at the metalacyclopropanc Xj extreme, the vinyl protons can resonate 5 ppm, and the vinyl carbons 100 ppm to high field of their position in the free ligand, as is appropriate for a change of hybridization from sp- to about SfP at carbon. Coordination shifts are usually much lower in the case of the L extreme. 

Strained alkenes, such as cyclopropene or norbomcne (5.5), bind unusually strongly to metals because the rehybridization on binding leads to relief of strain. Much of the strain in a small ring compound arises because the real 

C—C-C angles are constrained to be smaller than the ideal ones. Such an alkene is therefore less strained when complexed because the ideal angles at the vinylic carbons drop from the value of 120 , appropriate tor sp hybridization, to close to 109 , appropriate for sp hybridization. In some cases very strained alkenes are only stable in the complexed form. Nonconjugated dienes such as 1,5-cyclooctadiene (cod), and norbomadiene (nbd), can chelate to the metal and 

The applicable bonding model depends on the strength of the back donation. The Dewar-Chatt (D-C, 5.3) model holds for weak back bonding and the metalacyclopropane (MCP, 5.4) model for strong back bonding. Experimental structures can fall anywhere between the two extremes. For Zeise s salt and other intermediate oxidation state late metals, the D-C model fits best, while for Pt(0), the MCP model applies. Both cases are considered r structures. 

In the D-C extreme, the ligand predominantly acts as a simple L donor like PPh3, but in the MCP extreme, we have a cyclic X2 dialkyl, as if an oxidative addition of the C=C tt bond had taken place. In both cases, we have a 2e ligand on the covalent model, but while the D-C formulation (L), 5.3, leaves the oxidation state unchanged, the MCP picture (X2), 5.4, adds two units to the formal oxidation state. By convention, the D-C model is always adopted for the assignment of the formal oxidation state to avoid ambiguity, because there is no sharp boundary between the D-C and MCP extremes. 

TABLE 5.1 Dewar-Chatt versus Metalacyclopropane Bonding Models 

Vinyl C-H bonds Near coplanar with C=C Strongly folded back Near sp  

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B. Alkene and Alkyne Complexes 1. Mono-enes and Mono-ynes... 

Carbon dioxide reacts either with alkene and alkyne complexes of nickel(0) or with free alkenes and alkynes in the presence of nickel(O) complexes, giving five-membered cyclic nickel(II) complexes (equations 90 and 91).278... 

Table 12 The C—C Distances (A) of Alkene and Alkyne Complexes of Copper(I)...

Figure 8 The preparations and crystal structures of copper(I) alkene and alkyne complexes... 

An earlier review treated the organometallic derivatives, including alkyl, aryl, vinyl, alkynyl, carbonyl, carbene, alkene and alkyne complexes,16 and these will not be treated here (see Table 1 for other books and reviews on organogold chemistry). Two important articles dealing with [AuCl(CO)], including its structure and catalytic properties, have been published recently.97,381... 

Carbonyl, Phosphine, Alkene, and Alkyne Complexes with jr-Ligands 2... 

Metal Alkyls, Caibenes. Carbynes, and Carbides 655 Nonaromalic Alkene and Alkyne Complexes 662 Metallocenes 669... 

Negishi, E.-i., Takahashi, T. Alkene and alkyne complexes of zirconocene. Their preparation, structure, and novel transformations. Bull. Chem. Soc. Jpn. 1998, 71, 755-769. 

Table 12.4 Preparation of Platinum(O) Alkene and Alkyne Complexes [Pt(0)(un)L2] ...

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