The coordination chemistry of only Lewis-base (LB)-stabilized pnictogenylboranes EH2BH2 center dot NMe3 (E= P, As) towards Tl(I) salts has been studied. The reaction of Tl[BArCl] (BArCl=[B(3,5-C6H3Cl2)(4)]) with the corresponding pnictogenylborane results in the formation of [Tl(EH2BH2 center dot NMe3)][BArCl] (1a: E= P; 1b: E= As). Whereas the Tl ion in 1a/b is monocoordinated, the exchange of the weakly coordinating anion (WCA) in the Tl(I) salt leads to the formation of a trigonal pyramidal coordination mode at the Tl atom by coordination of three equivalents of EH2BH2 center dot NMe3 in [Tl-EH2BH2 center dot NMe3)(3)][WCA] (2a: E= P, WCA= TEFCl; 2b: E= As, WCA= TEF) (TEF=[Al{OC(CF3)(3)}(4)], TEFCl =[Al{(OC(CF3)(2)(CCl3)}(4)]). Furthermore, by using two equivalents of PH2BH2 center dot NMe3, a Tl(I)-mediated P P coupling takes place in CH2Cl2 as solvent resulting in [Me3N center dot BH2PH2PHBH2 center dot NMe3][WCA] (WCA= TEF, 3a; BArCl, 3b; TEFCl, 3c). In contrast, for the arsenic derivatives 1b and 2b, no coupling reaction is observed. The underlying chemical processes are elucidated by quantum chemical computations.