CRYSTAL-STRUCTURES AND SINGLE-CRYSTAL OPTICAL-ABSORPTION SPECTRA FOR 2 NEW POLYMORPHS OF TETRAKIS(MU-PIVALATO)DIMOLYBDENUM(II)

Two new polymorphs, β- and γ-Mo2[O2CC(CH3)3]4, were crystallized. The crystal structure of the β-form indicates an I2/c unit cell with a−c = 12.747 (3), 18.322 (3), and 11.369 (1) Å, β = 91.83 (l)°,and Z = 4. The crystal structure of the β-form indicates a Pbcn unit cell with a−c = 12.711 (3), 18.351 (5), and 11.413 (3) Å and Z = 4. Both structures have one-dimensional chains of molecules formed by weak two Mo…O bonds between adjacent molecules, a common feature of many dimolybdenum tetra-carboxylates. There is some indication of disorder of the methyl carbons in the tertiary butyl groups on the chelate rings not involved in intermolecular bonding. Single-crystal polarized absorption spectra are presented for the 1 l0 face for the β-form and the 110 face for the γ-form. The spectra of both forms possess highly resolved vibrational structure. In the β-form the spectra indicate three different sites with origins at 21 875, 21 900, and 21 950 cm−1, and in the γ-form there are two different sites with origins at 21 925 and 21 985 cm−1. The spectra indicate that this first absorption band for each form is a weak nondegenerate electric dipole allowed transition with comparable intensities arising from vibronic contributions of degenerate molecular vibrations. They are therefore assigned as the δ →δ* transition. There is evidence that the transition moment vectors have been shifted significantly away from the Mo-Mo bond direction. Long Frank-Condon progressions provide information about vibrational frequencies in the excited state. © 1990, American Chemical Society. All rights reserved.