Dimolybdenum Complexes Containing Pairs of Bridging Diphosphine and Carboxylate Ligands. Synthesis and Structures of trans ‐Mo 2 (O 2 CCH 3 ) 2 (μ‐dppa) 2 (BF 4 ) 2 and trans ‐Mo 2 X 2 (O 2 C(CH 2 ) n CH 3 ) 2 (μ‐dppa) 2 (n = 2, 10 or 12; X = Cl or Br; dppa = N,N ‐Bis(diphenylphosphino)amine)

The red complex trans ‐Mo 2 (O 2 CCH 3 ) 2 (μ‐dppa) 2 (BF 4 ) 2 , 1 , was prepared by reaction of [Mo 2 (O 2 CCH 3 ) 2 (CH 3 CN) 6 ][BF 4 ] 2 with dppa (dppa = Ph 2 PN(H)PPh 2 ) in THF. The reactions of Mo 2 (O 2 C(CH 2 ) n CH 3 ) 4 with dppa and (CH 3 ) 3 SiX (X = Cl or Br) afforded the complexes trans ‐Mo 2 X 2 (O 2 C(CH 2 ) n CH 3 ) 2 (μ‐dppa) 2 (X = Cl, n = 2, 2; X = Br, n = 2, 3; X = Cl, n = 10, 4 ; X = Cl, n = 12, 5 ). Their UV‐vis, IR and 31 P{ 1 H}‐NMR spectra have been recorded and the structures of 1, 2 and 3 have been determined. Crystal data for 1 : space group P2 1 /n, a = 12.243(1) Å, b = 17.222(1) Å, c = 13.266(1) Å, β = 95.529(1)°, V = 2784.1(6) Å 3 , Z = 2, with final residuals R = 0.0509 and Rw = 0.0582. Crystal data for 24CH 3 Cl 2 : space group P2 1 /n, a = 13.438(1) Å, b = 19.276(1) Å, c = 14.182(1) Å, β = 111.464(1)°, V = 3418.9(6) Å 3 , Z = 2, with final residuals R = 0.0492 and Rw = 0.0695. Crystal data for 3·4CH 2 Cl 2 : space group P2 1 /n, a= 13.579(1) Å, b = 19.425(1) Å, c = 14.199(1) Å, β = 111.881(2)°, V = 3475.6(7) Å 3 , Z = 2, with final residuals R = 0.0703 and Rw = 0.0851. Comparison of the structural data shows that the effect of the axial ligand on weakening the Mo‐Mo bond strength is X − > CH 3 CN > BF 4 − . The T m values are 121.7 °C for 2 , 111.1 °C for 3 and 91.5 °C for 5 , respectively.