Identifying the Hydrogenated Planar Tetracoordinate Carbon: A Combined Experimental and Theoretical Study of CAlH and CAlH.

The chemical curiosity "planar tetracoordinate carbon" (ptC) has greatly broadened one's knowledge of molecular bonding motifs apart from the traditional, tetrahedral, van't Hoff and LeBel's concept. Synthesized ptC examples have been reported either in the solid state or in the gas phase, where the ptC core is usually metalized or organometallized. Surprisingly, there has been no experimental report on hydrogenated ptC to date. A possible answer to this situation could be the "promiscuity" of hydrogen when binding to other elements, which frustrates the formation of stable ptC that is in competition with other structures. In this Letter, we for the first time identified two hydrogenated ptC species, CAlH and CAlH, based on a photoelectron spectroscopic and quantum chemical study. The favorable site-selectivity of hydrogen was shown to be the bridge of the Al-Al bond rather than the terminus of Al, manifesting the thermodynamic preference of the 17e/18e counting rule over the 15e/16e rule.