Three-membered rings carbocation stabilization

Sulfur-stabilized (3-carbocations (three-membered rings)... 

Carbenes bear a resemblance to carbocations in that there is an empty p orbital that can behave as an electron sink. However, a full orbital that can serve as an electron source is on the same atom. Trichloromethyl anion loses chloride, forming the reactive dichlorocarbene, a neutral, electron-deficient, electrophilic intermediate. Stabilization in dichlorocarbene results from the interaction of the full lone pair orbitals of chlorine with the empty p orbital of the carbene (Fig. 8.11). If the donors on the carbene are good enough, the carbene becomes nucleophilic. With few exceptions, carbenes react stereospecifically with double bonds to produce three-membered rings. 

The fragmentation is pulled by the enone (with some help from the acid) and pushed by the stability of a tertiary carbocation as well as the release of strain as the single bond that is fragmented is in a three-membered ring. 

Based on the previous analysis, 3-methyl-l-hexene reacts with mercuric acetate to give secondary carbocation intermediate 72. Is this carbocation unusual relative to previously discussed carbocations, such as 30 in Section 10.3 Note that mercury is a transition metal with d-orbitals that can donate electron density to the carbocation, stabilizing that positive center via what is known as back-donation (similar to what is observed with the halogens in the formation of halonium ions). Despite the manner in which is it drawn, this carbocation is not a three-membered ring, but rather a secondary carbocation that is stabilized by back-donation from the mercury atom. The dashed line (--) indicates significant coordination between the carbon and mercury. 

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