Reactivity of the Ni→B dative σ-bond in the nickel boratrane compounds [κ4-B(mimBut)3]NiX (X = Cl, OAc, NCS, N3): synthesis of a series of B-functionalized tris(2-mercapto-1-tert-butylimidazolyl)borato complexes, [YTmBut]NiZ