Electronic spectra of cyclotetramolybdenum(II) diynes

Low-temperature electronic-absorption spectra are reported for the tetranuclear Mo(II) clusters Mo4Cl8(PR3)(4), R=Et and Bu" (single crystals and dilute polymer films), and Mo4Br8(PBu3")(4) (polymer film). The results support a molecular-orbital model in which the delta-bonds of the constituent Mo-2 subunits (d(Mo-2)= 2.21-2.23 Angstrom) have been broken in the rectangular clusters with the formation of long-edge Mo-Mo single bonds (d(Mo-2) similar to 2.9 Angstrom). There is no significant visible absorption polarized parallel to the short bonds; that is, the prominent metal-metal polarized (1)(delta --> delta*) transition of quadruply bonded Mo2X4(PR3)(4) complexes is not observed. A very intense near-UV absorption band at 311 nm (epsilon 30 600 M-1 cm(-1)) for Mo4Cl8(PBu3")(4) is assigned to the dipole-allowed (1)(sigma' --> sigma'*) transition associated with the long Mo-Mo bonds. The many weaker absorption systems in the visible spectra, consisting exclusively of bands polarized either parallel to the long Mo-Mo bonds or perpendicular to the Mo-4 plane, are assigned to pi --> sigma'* transitions and/or forbidden sigma' --> sigma'* transitions. No electronic emission is observed, nor are transient-absorption signals with lifetimes of greater than 20 ns. Together with the absence of any photochemistry for visible irradiation, the results indicate that the excited states undergo rapid nonradiative decay to the ground state. (C) 1999 Elsevier Science S.A. All rights reserved.