Protonation-induced paramagnetism. Structures and stabilities of six- and seven-coordinate complexes of Os(II) in singlet and triplet states: A density functional study

Li, Yeh, and Taube in 1993 (J. Am. Chem. Sec. 1993, 115, 10384) synthesized a number of complexes which can be formally regarded as protonated Os(II) species. Some of these were paramagnetic, in contrast to the diamagnetism o f the closed shell 5d(6) Os(II) ions. This intriguing phenomenon is investi gated theoretically using density functional theory. The geometries, stabil ities, and electronic structures of a series of six- and seven-coordinate o smium complexes were studied in gas phase and aqueous solution using the B3 P86 functional, in conjunction with the isodensity-polarized continuum mode l of solvation. The general formula for these complexes is [Os(NH3)(4)H(L-1 (x))(m)(L-2(y))(n)]((x+y+3)+), where L-1 and L-2 = H2O, NH3, CH3OH, CH3CN, Cl-, and CN-, which could be regarded as protonated Os(II) species or hydri des of Os(IV), although according to this work the osmium-hydrogen interact ion is best described as a covalent Os(III)-H bond, in which the hydrogen i s near-neutral. The ground states are generally found to be singlets, with low-lying triplet excited states. Solvation tends to favor the singlet stat es by as much as similar to 18 kcal mol(-1) in the 3+ ions, an effect which is proportional to the corresponding difference in molecular volumes. To h ave realistic estimates of the importance of spinorbit coupling in these sy stems, the spin-orbit energy corrections were computed for triplet [Os(NH3) (4)](2+), [Os(NH3)(4)H](3+), and [Os(NH3)(4)H(H2O)](3+), along with gas-pha se Os and its ions as well as [Os(H2O)(6)](3+). The seven-coordinate triple t-state complex [Os(NH3)(5)H(CH3OH)(3+), which had been successfully isolat ed by Li, Yeh, and Taube, is predicted to be a stable six-coordinate comple x which strongly binds to a methanol molecule in the second coordination sh ell. The calculations further suggest that the singlet-triplet splitting wo uld be very small, a few kilocalories per mole at most. The geometries and the electronic structures of the complexes are interpreted and rationalized in terms of Pauling's hybridization model in conjunction with conventional ligand field theory that effectively precludes the existence of true seven -coordinate triplet-state complexes of the above formula.