“NON-CLASSICAL”CARBOCATIONS—ISOELECTRONIC AND ISOSTRUCTURAL CARBON COPIES OF POLYBORANES; CONSERVATION OF CHEMICAL SHIFT

Abstract

The 13C chemical shifts of the five skeletal carbon atoms in the carbocation 1,2-Me2C5H3+ may be compared to the 11B chemical shifts of the corresponding boron atoms in the polyborane nido-1,2-Me2B5H7 via the relationship:δ11BBF3:OEt2=0.33 δ13CTMS−which is similar to that reported previously by Spielvogel et al. and also by Nöth and Wrackmeyer. The close agreement between the observed and calculated values leaves no doubt that the two compounds are isoelectronic and isostructural. Similarly, other“non-classical”carbocations may be compared to isoelectronic and isostructural polyboranes: the cluster atoms in Me6C62+ (a nido compound) are directly comparable to those in nido-B6H10; the three critical carbons in 2-norbornyl cation (an arachno species) are comparable to the borons in arachno-B3H7:L; the carbon atoms in cyclopropylcarbinyl cation (bicyclobutyl cation) can be related to the boron atoms in arachno-B4H9−x; each of the three carbon atoms in the critical cluster of tris-homocyclopropenium cation and its derivatives (hypho-compounds) can be compared to the BH2:L moiety in B3H7:L and the five cluster carbons in the alkyl derivatives of C5H9+ are analogous to the borons in hypho-B5H9:L2. All of the data support the proposal that the cations investigated are non-classical and are simply“carbon-copies”of their polyborane analogues.