Bonded alkyl, aryl and related ligands

In complexes such as WMe, [MoMey] , TiMc4 and MeMn(CO)5, the M—Cmc bond can be described as a localized 2c-2e interaction, i.e. it parallels that for the [r -Cp] ligand (see Box 19.2). The same bonding description is applicable to the Fe-Cph bond in 24.2 and the Fe—CcHO bond in 24.3. 

The interplay of donation and back-donation of electronic charge between a metal and rr-acceptor ligand is an example 

In multinuclear metal species, CO ligands may adopt terminal (24.5) or bridging (24.6 and 24.7) modes. Other modes are known, e.g. semi-bridging (part way between 24.5 and 24.6) and mode 24.8. 

An analysis of more than 20000 crystal structures of J-block metal carbonyl complexes confirms a clear correlation between C O and M-C bond distances, i.e. as the M-C bond distance decreases, the C bond distance (rfco) increases. Ninety per cent of the structural data fall into a region in which 117.0 pm dco 112.8 pm, and for these M-CO interactions, the M-C t- and w-bonding contributions are approximately in balance. For 4%, w-bonding dominates and dco 117.0 pm, while for 6%, cr-bonding and ionic contributions dominate and dco 112.8 pm. 

CO ligands shift to lower frequency, e.g. in the C Self-study exercises 

Carbonyl ligand environments can also be investigated using C NMR spectroscopy, although systems are often fluxional (e.g. Fe(CO)5, see Fig. 4.24 and discussion) and information about specific CO environments may flierefore be masked. Some useful points are that  

In keeping with the typical weakening of the C—O bond on going from free CO to coordinated CO, X-ray diffraction data show a lengthening of the C O bond. In CO, the C—O bond length is 112.8 pm, whereas typical values in metal carbonyls for terminal and p-CO are 117 and 120 pm respectively. 

Carbonyl hgand environments can also be investigated using C NMR spectroscopy, although systems are often 

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METAL ALKYLS. ARYLS. AND HYDRIDES AND RELATED o-BONDED LIGANDS... 

Structural parameters of selected mononuclear complexes containing heteroatomic 7i-donor-substituted allenylidene ligands are summarized in Table 1. When these data are compared to those reported for related complexes bearing conventional alkyl or aryl substituents (see Table 1 of chapter All-Carbon-Substituted Allenylidene and Related Cumulenylidene Ligands ), a lengthening of the M=C bond... 

Apart from the platinum and nickel compounds with cr-bonded alkyl or aryl ligands, Chatt s group reported between 1961 and 1966 the synthesis of an impressive series of related cobalt(II), iron(II), rhodium(III), iridium(III), rhe-nium(III) and rhenium(V) complexes [39-42]. The paramagnetic cobalt(II) and iron(II) derivatives MR2(PR 3)2 were stable when R was an aryl group with... 

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