Mixed Chloride/Amine Complexes of Dimolybdenum(II,II). 6. Stepwise Substitution of Amines by Tertiary Phosphines and Vice Versa: Stereochemical Hysteresis

The substitution reactions of primary amines by tertiary phosphines in quadruply bonded dimolybdenum(II,II)
complexes Mo2Cl4(NH2R)4 have been studied. The exchange reaction has been shown to result at room temperature
in disubstituted species Mo2Cl4(NH2R)2(PR3)2 (PR3 = PMe3, NH2R = NH2Prn (1a), NH2But (2a), NH2Cy (3a);
PR3 = PMe2Ph, NH2R = NH2Cy (4a)), while heating is needed to obtain fully substituted complexes Mo2Cl4(PR3)4. The crystal structure of disubstituted products has been investigated by X-ray crystallography and revealed
that they all belong to the α-isomer, having both phosphine groups at the same Mo atom. Crystal data are as
follows:  for 1a, tetragonal space group I41/a with a = 17.737(2) Å, c = 15.6915(6) Å, and Z = 8; for 3a,
monoclinic space group P21 with a = 10.963(3) Å, b = 10.117(2) Å, c = 13.323(4) Å, β = 90.05(2)°, and Z =
2; for 4a, triclinic space group P1̄ with a = 9.329(3) Å, b = 10.206(2) Å, c = 18.975(3) Å, α = 85.45(2)°, β =
87.10(1)°, γ = 80.88(1)°, and Z = 2. The substitution processes for the direct and reverse reactions have been
monitored by 31P NMR. They both proceed in a stepwise manner; however, a stereochemical hysteresis is taking
place, i.e., the back reaction, the substitution of phosphines by amines, goes through another isomer of Mo2Cl4(NH2R)2(PR3)2, having phosphine ligands on different Mo atoms. This β-isomer is more thermodynamically stable
and can be obtained by thermal conversion of the α-form. All chemical equilibria studied in the paper have been
explained as governed by a higher trans effect of PR3 groups compared to NH2R groups.