Pentacoordinate hypercarbon

The norbornyl ion controversy """ centered on the question of whether the ion had the static bridged nonclassical structure (126) containing pentacoordinate hypercarbons or whether its structure was better depicted as a rapidly equilibrating pair of trivalent classical ions (127a and 127b). 

The first clear example of cyclometallation of sp C-H bonds was shown by Chart and Davidson" and Cotton and coworkers in the case of the ruthenium complex [Ru(dmpe)2] [dmpe = l,2-bis-(dimethylphosphino)ethane]. This complex was found to spontaneously cyclometallate the C-H a bond of the methyl group on the phosphorous through a pentacoordinate hypercarbon intermediate to provide a dimer"" [Eq. (6.62)]. Tlie same complex also reacts with the aromatic C-H bond of naphthalene... 

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