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.
A (13)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 several nido and closo carboranes containing five and six coordinated carbons and their (13)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 the (13)C chemical shifts of five- and six-coordinated carbons, compared with those of four valent carbons, with a simultaneous general increase of J(C-H) coupling constants.
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106.
G A
GA
Olah
G K
GK
Prakash
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G
Liang
K L
KL
Henold
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GB
Haigh