The electronic structures and spectroscopic properties of a series of trans-dioxoosmium(VI) complex ions
[OsO2(L)4]z (L = CN, z = 2- (1); L = CO, z = 2+ (2); L = CCH, z = 2- (3)) were investigated using quantum chemistry
methods. The geometrical structures of the complexes were fully optimized at the B3LYP level for the ground state and
the CIS level for the lower-lying excited state with the LANL2DZ basis sets with two f-type (for Os) and two d-type (for
C, N, and O) polarization functions, respectively. The calculations revealed that the bond lengths of Os=O in the excited
state are elongated relative to those in the ground state for the three complexes. The calculation results show that
the dipole-allowed absorptions of the three complexes are in the region of λ >230 nm. The high energy absorptions at
353 nm for 1, 359 nm for 2, and 314 nm for 3 are mainly assigned as π (CN)→π*(Os=O), px(O)→σ*(Os-CO), and
π(CC)→π*(Os=O), respectively. Whereas the lower-lying absorptions at 531 nm and 475 nm for 1 and 3, respectively, are
originated from metal centered d-d transition combined with ligand (CN and CCH) to ligand (O) charge transfer character,
but that at 413 nm for 2 due to [px,y(O)→π(dxz,yz-px,y)(Os=O)]. The phosphorescence was calculated with the TD-DFT
method based on the optimized geometry structure in the excited state. The calculated phosphorescence at 617 nm 638 nm
and 553 nm originate from 3[(π*(Os=O))1(Os(dxy)+π(CN/CCH))1] and 3[(π*(Os=O))1(pz(O))1] excited states for 1/3 and 2,
respectively. The phosphorescence energies are in the order of 2>1>3, therefore the emission can be influenced and tuned
by the attached ligands.
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