Protonation and alkylation of (Idipp)Si=Si-(Idipp) (1) afforded the mixed-valent disilicon(I)-borates [(Idipp)(R)Si-II=S-0 (Idipp)][B(Ar-F)(4)] (IR[B (Ar-F)(4)]; R = H, Me, Et; Ar-F = C6H3-3,5-(CF3)(2); Idipp = C[N(C6H3-2,6-iPr(2))CH](2)) as red to orange colored, highly air-sensitive solids, which were characterized by single-crystal X-ray diffraction, IR spectroscopy and multinuclear NMR spectroscopy. Dynamic NMR studies in solution revealed a degenerate isomerization (topomerization) of the "sigma-bonded" tautomers of 1H[B-(Ar-F)(4)], which proceeds according to quantum chemical calculations via a NHC-stabilized (NHC = N-heterocyclic carbene) disilahydronium ion ("pi-bonded" isomer) and is reminiscent of the degenerate rearrangement of carbenium ions formed upon protonation of olefins. The topomerization of 1H[B(Ar-F)(4)] provides the first example of a reversible 1,2-H migration along a Si=Si bond observed in a molecular system. In contrast, 1Me[B(Ar-F)(4)] adopts a "rigid" structure in solution due to the higher energy required for the interconversion of the "sigma-bonded" isomer into a putative NHC-stabilized disilamethonium ion. Addition of alkali metal borates to 1 afforded the alkali metal disilicon(0) borates 1M[BAr4] (M = Li, Ar = C6F5; M = Na, Ar = Ar-F) as brown, air-sensitive solids. Single-crystal X-ray diffraction analyses and NMR spectroscopic studies of 1M[BAr4] suggest in concert with quantum chemical calculations that encapsulation of the alkali metal cations in the cavity of 1 predominantly occurs via electrostatic cation-pi interactions with the Si=Si pi-bond and the peripheral NHC aryl rings. Displacement of the [Si(NHC)] fragments by the isolobal fragments [PR] and [SiR](-) interrelates the cations [(NHC)(R)Si=Si(NHC)](+) to a series of familiar, multiply bonded Si and P compounds as verified by analyses of their electronic structures.