A 13C 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. 13C nuclear magnetic resonance shifts of several nido and closo carboranes containing five and six coordinated carbons and their 13C-IH 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