Chemical detection of carbon-metal bonds: From distinct sigma-bonding of main-group metals to ambiguous pi-bonding with transition metals

The detection, site of binding and quantification of bonds between carbon and a main-group metal can be readily achieved by a combination of proto- and deuteriodemetallation of such stable organometallics, combined with NMR spectral and mass spectrometric analyses of the organic products. Only with chiral sp³-carbon-metal bonds, with geometrically isomeric sp²-carbon-metal bonds or with allylic carbon-metal bonds will further structural physical data be required to identify the actual 3D-structure of the carbon binding site.The thermal and photochemical lability of carbon bonds to transition metals imposes restrictions on the detection of C−M_t bonds by deuteriodemetallation. Often the homolytic rupture of such bonds competes greatly and will lead to geometric isomers. Results of such apparent protodemetallation should be cross-checked with other structural information.The detection of C=M_t and possibly C≡M_t bonds may prove to be generally achievable by cycloadditions of such metal carbenes or carbynes with nitriles or alkynes.The addition of low-valent transition metal salts to alkenes or alkynes leads to complexes viewable as pi-complexes or as epimetallated adducts. By examination of available structural parameters and the chemical reactions undergone by such adducts, the adducts between ethylene and Ti(OiPr)₂ and between diphenylacetylene and α,α-bipyridyl-nickel(0) can best be considered as titana(IV)cyclopropane and nickel(II)cyclopropene structures, respectively.