The synthesis and tautomerization of a "half-parent" aminosilylene and its heavy P- and As-analogues ((LSi)-L-TMS-EH2; E = N, P, As; L-TMS = N(SiMe3)(2,6-(Pr2C6H3)-Pr-i)) in the coordination sphere of nickel(0) to give the corresponding side-on eta(2)-RSi(H)=EH and RH2Si-E ("silylpnictinidene") nickel complexes are reported. These complexes can be accessed through salt metathesis reactions of the lithium dihydropnictides LiEH2 with the acyclic chlorosilylene nickel(0) complex 1, [L-TMS(Cl)Si -> Ni(NHC)(2); NHC = :C[(Pr-i)NC(Me)](2)). In addition, we report the facile E-H bond activation reactions of EH3 with 1, which furnished a silyl nickel(II) complex through NH3 activation, but phosphido and arsenido complexes in the activation of PH3 and AsH3, respectively. Notably, reaction of 1 with LiNH2 leads to the acyclic bis(amido)silylene complex [L-TMS(H2N)Si -> Ni(NHC)(2)] 5, which does not undergo N-H proton migration to silicon(II) under ambient conditions. The transformation of the P- and As-analogues of 1 furnishes directly the respective side-on Si=E Ni complexes (nickelacycles), [eta(2)-{L-TMS(H) Si=E(H)}Ni(NHC)(2)] (E = P, 6; E = As, 9). These nickelacycles show a vastly different stability in solutions. While 6 is stable for several days at ambient temperature, 9 undergoes further rearrangement processes within minutes of its formation. Given the high acidity of the As-H proton in 9, however, this moiety can be trapped as a highly charge separated metalated-eta(2)-silaarsene nickel complex 12 that is best described as an [AsSiNi] nickelacycle with Si-As multiple bond character. Taken as a whole, these results give, for the first time, insights into the relative stability of the tautomeric forms of side-on silaldimine transition metal complexes. The electronic nature and the rearrangement processes of these compounds were also investigated by quantum chemical calculations.