SYNTHESIS AND CHARACTERIZATION OF DIHYDROGEN COMPLEXES OF TETRACARBONYLBROMOMANGANESE(I) AND TETRACARBONYLCHLOROMANGANESE(I) IN ARGON MATRICES

When argon matrices of MnX(CO)(5), X = Br and Cl, containing dihydroge n are photolyzed with either visible Light or light of 254 nn, a new s pecies is formed which is MnX(eta(2)-H-2)-(CO)(4). The compounds have been characterized by infrared and Raman spectroscopies, and an energy -factored force field has been fit to the carbonylmodes. The structure s can be thought of as arising fi om an octahedron with no halide and dihydrogen occupying cis-positions. While the H-H stretching vibration was not observed, the symmetric metal-hydrogen stretches have been ob served at 788.6, 665.2, and 588.5 cm(-1) for the adducts of H-2, HD, a nd D-2 and MnBr(CO)(4), respectively. The same mode occurs at 764 cm(- 1) for MnCl-(H-2)(CO)(4); hence, H-2 is not strongly bound to the meta l. An analysis of the carbonyl stretching force constants supports the view that H-2 is not as strong of a pi-acid as CO. The spectra of MnX (CO)(4) have been re-examined, and it is clear that the features that had been previously assigned to MnX(CO)(4) of C-2v symmetry actually b elong to two different species. The spectra are consistent with one is omer of low symmetry, presumably due to the loss of an equatorial CO f rom MnX(CO)(5). The other isomer gives only a single strong infrared a bsorption and could be the isomer formed with the loss of an axial CO from MnX(CO)(5).