13 C nuclear magnetic resonance study of five- and six-coordinated carbon in nonclassical organometallic compounds: Dimeric trialkyl-, tricyclopropyl-, and triarylaluminums and some nido and closo carboranes

A13 C nuclear magnetic resonance spectroscopic study of dimeric trimethyl-, triethyl-, tricyclopropyl-, and triarylaluminums is reported. The five-coordinated bridging carbons are found consistently more shielded than the terminal carbons, in accordance with the increased p-character of the former. The nature of bridging two-electron three-centered Al—C—Al bonds is discussed.13 C nuclear magnetic resonance shifts of severalnido andcloso carboranes containing five and six coordinated carbons and their13 C-1 H spin-spin coupling constants were also obtained. The relationship between the carbon chemical shifts and coordination number of the carbon atom is discussed. There is approximately a 20- to 40-ppm shielding of the13 C chemical shifts of five- and six-coordinated carbons, compared with those of four valent carbons, with a simultaneous generalincrease of JC-H coupling constants.