The gist of the proposal is that a carbanion or carbocation absent a good leaving group can be chelated by Hydride Sponge or Proton Sponge, respectively, to give a stable isolable 5-coordinate carbon atom.  Simple systems like -C(CN)3, -C(COOMe)3, and -C(CONMe2)3 look good on paper (HyperChem Lite)and with mathematical modeling.  Elegant dipolar systems offer alternate reaction paths like electron redox to tar against slow kinetics of chemical combination to desired product. Intuition only takes us as far as Pyrex.  Observation defines common sense.  This last example is a very sterically crowded adduct of Proton Sponge + tetracyanoethylene + Hydride Sponge to give the dipole, http://www.mazepath.com/uncleal/tcne.png We see severe distortion of the rings.  This is in part due to severe crowding by the methyls (hydrogens not shown but included in the mm3 minimalization) plus equatorial coordination of the chelate.  Unlike the Meldrum's acid case, both pentacoordinate carbons are beautiful trigonal bipyramids.  When I have time I'll do the axial chelate case and compare energies. Uncle Al therefore proposes the following:    1) A facile general route to isolable pentacoordinate carbon may be available.    2) The best route appears to be Hydride Sponge, bis-1,8-(dimethylboro)naphthalene, plus a rather basic carbanion lacking a nucleofuge group and with three untethered subsitutents in a dipolar aprotic solvent, probably with the accompanying cation encased in a crown or crypt ether.    3) The kinetics of combination will probably be very slow.  It is therefore important that the carbanion be stable in the reaction mixture over weeks or months.    4) Candidate carbanions are -C(CN)3, -C(COOMe)3, -C(CONMe2)3, and possibly even dimethyl (or di-t-buytl) malonate carbanion. http://www.mazepath.com/uncleal/rawmat.png  Starting materials http://www.mazepath.com/uncleal/product.png  Elegant, but probably horribly slow kinetics vs. redox tar instead http://www.mazepath.com/uncleal/cn3.png  Simple is good http://www.mazepath.com/uncleal/amindo5.png  More basic carbanion; no good leaving group http://www.mazepath.com/uncleal/guanid.png  Probably too delocalized. http://www.mazepath.com/uncleal/meldrum.png  With Meldgrum's acid carbanion; distorted