Two series of novel heteroleptic derivatives of lanthanides and (η6-benzoic acid)tricarbonylchromium (benchrotrenecarboxylic acid) were synthesized and characterized: mononuclear complexes [Ln(BcrCOO)(acac)2(H2O)2] (Ln = Eu (1), Gd (2), Tb (3a), Dy (4a), Ho (5a)) and 1D-polymeric ones [Ln(BcrCOO)(acac)2(H2O)]n (Ln = Tb (3b), Dy (4b), Ho (5b), Er (6), Tm (7), Yb (8) and Y (9)), Bcr = (η6-C6H5)Cr(CO)3. The molecular and crystal structures of the obtained compounds were determined. Complexes 3a, 4a, 4b, 6 and 8 were found to exhibit SMM properties. Two maxima were observed on χ''(ν) dependencies (LF and HF) for 4a, 4b and 6. For 4a, the anisotropy barriers are Δeff/kB = 100 K and 118 K in zero dc-field (128 K and 143 K in 2000 Oe field) for the LF (low frequency) and HF (high frequency) signals, respectively. These values are the highest ones for Ln carboxylate complexes. The nature of the appearance of two maxima in the system is discussed.