CONVERSION OF DINUCLEAR, DIAMAGNETIC [MO(V)(2)O-3(NCS)(8)](4-) INTO MONONUCLEAR, PARAMAGNETIC [MO(V)O(NCS)(5)](2-) THROUGH PHOTOLYTIC LIGAND REDISTRIBUTION

Dinuclear [(Mo2O3)-O-V(NCS)(8)](4-)(1(4-)) has been prepared as [NMe4] (+) and [NEt4](+) salts in good yield by reduction of [(MoO2)-O-VI(NCS )(4)](2-) with 0.5 equiv of PMe2Ph in tetrahydrofuran. A Beer's law pl ot of the band at 513 nm (epsilon=22 500 mol(-1) L cm(-1)) from concen trations of 1 x 10(-4)-1 x 10(-3) mol L-1 established that 1(4-) does not participate in thermal disproportionation equilibria with mononucl ear Mo(VT) and Mo(TV) species, A single crystal diffraction study has established that dark crystals containing 1(4-) and a noncoordinating [NMe4](+) counterion crystallize from acetone as the solvate [NMe4](4) [(Mo2O3)-O-V(NCS)(8)].2Me(2)CO in the triclinic space group P (1) over bar with a=9.455 (2) Angstrom, b=11.831 (2) Angstrom, c=12.772 (3) An gstrom, alpha=110.52 (3)degrees, beta=105.17 (3)degrees, gamma=98.63 ( 3)degrees, and Z=1. The anion in these crystals has an anti orientatio n of the terminal oxo groups similar to that observed previously for a hydrate of the K+ salt. Ion pairing in the K+ salt has surprisingly m odest effects on anion parameters for the anion, largely confined to i ncreases in the bending at N of the isothiocyanate ligands. The tetraa lkylammonium salts are photosensitive in CH3CN, and purple solutions b ecome orange within hours under ambient lighting. Disappearance of the absorbance at 513 nm and appearance Of a new visible absorbance at 45 5 nm is accompanied by well-defined isosbestic points at 483, 394, 367 , and 322 nm. The photolysis product is paramagnetic (mu(eff)=1.86 BM in solution) and has been shown to be mononuclear [(MoO)-O-V(NCS)(5)]( 2-) by comparison (UV/visible, IR, combustion analysis, EPR) with a re ference sample. Reproducible synthetic photolysis of [(Mo2O3)-O-V(NCS) (8)](4-) requires an atmosphere of O-2, consistent with an initial pho tochemical redistribution of the isothiocyanate ligands to form [(MoO) -O-V(NCS)(5)](2-) and a Mo(V) oxo complex which is then oxidized. The low disappearance quantum yield in the visible (Phi(lambda>460)=1.3x10 (-4)) establishes that the reaction is not connected with the bridging oxo chromophore but probably involves initial NCS-dissociation follow ing UV excitation (Phi(310)=1.4x10(-3) under N-2).