Studies of distortional isomers: Spectroscopic evidence that green cis,mer-dichlorotris(dimethylphenylphosphine)oxomolybdenum(iv) is a mixture

Article abstract of DOI:10.1021/ja00024a027

The blue (1) and green (2) forms of cis,mer-MoOCl₂(PMe₂Ph)₃ are not distortional isomers. An authentic sample of green 2 has been chromatographically and spectroscopically characterized to be a 65:35 mole fraction mixture of blue cis,mer-MoOCl₂(PMe₂Ph)₃ and yellow mer-MoCl₂(PMe₂Ph)₃ (yellow 3), respectively. Thin-layer chromatography separates green 2 into two individual peaks due to blue 1 and yellow 3. Elemental analysis of green 2 shows a significantly higher percent chlorine relative to blue 1, consistent with the presence of a 0.35 mole fraction of yellow 3 in green 2. Both infrared (500-150 cm^-1) and Raman (1625-125 cm^-1) spectra of green 2 have distinct features due to contributions from blue 1 and yellow 3. Furthermore, Raman spectra of blue 1 and green 2 indicate a ν(Mo=O) band at 944 ± 1 cm^-1 that is conspicuously absent in the spectrum of yellow 3. The 500-MHz ¹H NMR spectrum for green 2 shows several resonances characteristic of paramagnetic yellow 3 in the regions +7.5 to +11.5 ppm and -22 to -41 ppm. Visible spectroscopy implies that the color of green 2 is due to the summation of absorptions from blue 1 and yellow 3. X-ray photoelectron spectroscopy (XPS) of the Mo 3d regions resolves the contributions from the Mo(IV) (blue 1) and Mo(III) (yellow 3) species in green 2. Both the visible and XPS results confirm the 65:35 (blue1:yellow 3) molar composition of green 2. The facile cocrystallization of blue 1 and yellow 3 to give green 2 is likely driven by the increased entropy of the green 2 crystals, which decreases the free energy of crystallization. Similar compositional disorder processes are probably responsible for the anomalous bond lengths ascribed to distortional (bond stretch) isomerism in other series of inorganic complexes.