M-L Multiple Bonds

We now look in detail at compounds with multiple bonds to C, N, and O, particularly carbene and carbyne complexes, L M=CR2 and L M=CR, which at least formally contain M=C or M=C multiple bonds. 

A free carbene such as CH2 has two spin states, singlet (ft) and triplet (It) that are distinct spin isomers with different reactivities and structures. In the singlet, the electrons are paired up in the sp lone pair, but the triplet has one electron in each of the spi and p orbitals (Fig. ll.lfl). Unlike many of the ligands discussed previously, carbenes are rarely stable in the free state. Methylene, CH2, for example, is a transient intermediate that even reacts with alkanes. This instability—both thermodynamic and kinetic—contributes to its very strong metal binding by disfavoring carbene dissociation. 

The two main types of coordinated carbene are named after their discoverers Fischer and Schrock. Each represents an extreme formulation 

The Organometallic Chemistry of the Transition Metals, Sixth Edition. Robert H. Crabtree. 

FIGURE 11.1 Singlet and triplet forms of a carbene (a) can be considered as the parents of the Fischer (h) and Schrock (c) carbene complexes. In the Fischer case, direct C- M donation predominates, and the carbon tends to be positively charged. In the Schrock case, two covalent bonds are formed, each polarized toward the carbon, giving it a negative charge. 

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H2 addition across M-L multiple bonds is not as common but has been studied... 

Polyfluoroalkyl- andperfluoroalkyl-substituted CO and CN multiple bonds as dipolarophiles. Dmzo alkanes are well known to react with carbonyl compounds, usually under very mild conditions, to give oxiranes and ketones The reaction has been interpreted as a nucleophilic attack of the diazo alkane on the carbonyl group to yield diazonium betaines or 1,2,3 oxadiazol 2 ines as reaction intermediates, which generally are too unstable to be isolated Aromatic diazo compounds react readily with partially fluorinated and perfluorinated ketones to give l,3,4-oxadiazol-3-ines m high yield At 25 °C and above, the aryloxa-diazolines lose nitrogen to give epoxides [111]... 

Certain general trends in M-0 bond distances can be correlated with bond multiplicity and the degree of oxygen-to-metal TT-bonding. A comparison of M-0 bond distances for structurally related compounds having a M, (p3-X) 2 (p2-X) Xx 0 unit is given in Table II. For M = Mo and W in closely related compounds, differences in M-L distance are negligible. 

M—L bonds in d° systems may be authentic (e.g. equation 19). Elimination could still take place from an t]1-H2 complex because H2 can add in this way without requiring metal d electrons to be available. Molecular hydrogen can also give metal hydrides by more complicated processes, e.g. equation (20), involving ligand hydrogenation and multiple H2 additions. The formation of hydrides from H2 is involved in the catalysis of hydrogenation, hydroformylation and isomerization by metal... 

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