The complexes [Ln(DippForm)2(QAr)(THF)] [DippForm = N,N′-(2,6-diisopropylphenyl)formamidinate; Ln = Sm, Yb; QAr = SPh, SePh, TePh, SeC6F5, Se(p-C6F4OMe)] were synthesized by reduction of corresponding dichalcogenides Ar2Q2 with the LnII formamidinates, [Ln(DippForm)2(THF)2]. All complexes with QPh ligands have similar molecular structure (determined by X-ray diffraction), they are mononuclear due to the effective shielding of the central atom by the bulky amidinate ligands, and possess transoid disposition of the latter. The complexes with fluorinated phenyl groups (ArF) possess an additional coordination of an o-F atom to the lanthanide ion. Only the compounds [Yb(DippForm)2(QArF)(THF)] easily lose the coordinated THF due to the sterically crowded coordination sphere, and the addition of THF is reversible even at ambient temperature. The Sm compounds are stable towards elimination of THF upon moderate heating. Similar crystal packing is found for most of the complexes, except for [Sm(DippForm)2(SPh)(THF)], [Yb(DippForm)2(SeC6F4OMe)(THF)] and [Yb(DippForm)2(SeC6F5)], which was rationalized as a consequence of interaction of FormDipp ligands from neighboring molecules to form intermolecular junctions. Analogous junctions were found in the majority of known complexes with Ln(DippForm)2 moiety (for all lanthanides). In the IR spectra the bands characteristic to the C–N, C–F and C–O bonds were revealed, some of which are noticeably influenced by the type of co-ligands.