  We study the structural dynamics of photoexcited [Co(terpy)2]2+ in an aqueous
solution with ultrafast x-ray diffuse scattering experiments conducted at the
Linac Coherent Light Source. Through direct comparisons with density functional
theory calculations, our analysis shows that the photoexcitation event leads to
elongation of the Co-N bonds, followed by coherent Co-N bond length
oscillations arising from the impulsive excitation of a vibrational mode
dominated by the symmetrical stretch of all six Co-N bonds. This mode has a
period of 0.33 ps and decays on a subpicosecond time scale. We find that the
equilibrium bond-elongated structure of the high spin state is established on a
single-picosecond time scale and that this state has a lifetime of ~ 7 ps.
