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The coordination complex [Mn(H2O)6]2+ would exhibit the least intense peaks in the UV-Vis spectrum. This is because Mn(II) has a d5 electronic configuration, making it a high-spin complex with no d-d transitions in the visible region. The other complexes ([Ti(H2O)6]2+, [V(H2O)6]2+, and [Ni(H2O)6]2+) possess d-electrons that can undergo d-d transitions, leading to absorption in the visible region and consequently, more intense peaks in the UV-Vis spectrum.