A series of Ln(II) and Ca(II) bis(alkyl) complexes with bulky benzhydryl ligands, [(p-tBu-C6H4)(2)-CH](2)M(L-n) (M = Sm, L = DME, n = 2 (1); M = Sm, Yb, Ca, L = TMEDA, n = 1 (2, 3, 4), were synthesized by the salt-metathesis reaction of MI2(THF)(n) (n = 0-2) and [(p-tBu-C6H4)(2)CH]Na--(+). In complex 1, the benzhydryl ligands are bound to the metal center in eta(2)-coordination mode. Unlike complex 1, in isomorphous complexes 3 and 4, due to the coordination unsaturation of the metal center, the both benzhydryl ligands coordinate to the metal in eta(3)-fashion. In complex 2, one ligand is eta(3)-coordinated while the second one is eta(4)-coordinated to the Sm(II) ion. Complexes 2-4 demonstrated unprecedented thermal stability: no evidence of decomposition was observed after heating their solutions in C6D6 at 100 degrees C during 72 h. Complex 1 behaves differently: thermolysis in C6D6 solution at 75 degrees C results in total decomposition in 8 h. Addition of DME promotes decomposition of 2-4 and makes it feasible at 40 degrees C. Complexes 1-4 demonstrated high catalytic activity and excellent regio- and chemoselectivities in intermolecular hydrophosphination of double and triple C-C bonds with both primary and secondary phosphines. Complexes 2 and 3 enable addition of PhPH2 toward the internal C=C bond of Z- and E-stilbenes with 100% conversion under mild conditions. Double sequential hydrophosphination of phenylacetylene with Ph2PH and PhPH2 was realized due to the application of Yb(II) complex as a catalyst.