Can C-H...C(p) bonding be classified as hydrogen bonding? A systematic investigation of C-H...C(p) bonding to cyclopentadienyl anions.
The following ammonium and phosphonium cyclopentadienide complexes were prepd., crystd. and studied structurally: [Et4N]+[Cp]-, [Et4P]+[Cp]-, [Ph3PMe]+[Cp]-, [Ph2PMe2]+[Cp]-, [Ph4P]+[tBu(Me)Cp]- and [Et4P]+2[CpCMe2CMe2Cp]2-. The Cp anions in these complexes can not be considered as naked. Intermol. bonding through characteristic (cation)C-H...C(Cp-) interactions gave polymeric networks. Generally, polymeric chains of alternating cations and anions are formed that are reminiscent of the solid-state structures obsd. for cyclopentadienyl-alkali metal compds. Mutual comparison of all the known structures of ammonium and phosphonium cyclopentadienide complexes shows that intermol. C-H...C(Cp-) bonding displays H bond-like characteristics: (i) directionality, (ii) correlation between C-H acidity and C-H...C bond lengths and (iii) correlation between the strength of the H bond acceptor and C-H...C bond lengths. These observations are also confirmed by MP2(fc) ab initio calcns. on coordination complexes of C-H donors (acetylene and methane) and the p-systems in benzene or the Cp- anion. Likewise, combinations of an alkali metallocene anion and a phosphonium cation show such C-H...C(Cp-) interactions in the cryst. state.
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