Carbon, planar tetra coordinate

The structural unit [CAU] " is reported in the salt (Na )2[CAl4] . The product is made by laser vaporisation of an Al/C/Na composite target containing 75% Al, 5% C and 20% Nal by mass. A time-of-flight mass spectrometer was used to select specific ions and the product was analysed by photoelectron spectroscopy backed up by theoretical calculations. It contains a planar tetra-coordinate carbon atom. Such a pentaatomic unit is, of course, the smallest structural unit that may contain such an atom, making this product a compound... 

Significant theoretical and experimental efforts to get a planar arrangement of atoms belonging to representative elements, which are included in simple and elaborate molecular entities, appeared in the last decade. Along this line, an interesting structural pattern 16a-r has been presented by the group of Li, where planar central silicon and carbon atoms with tetra-, penta-, hexa-, and heptacoordination numbers were found for the former and tetra- and penta-coordination numbers for the latter <2004JA16227>. 

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