Complexes 1-M range in color from pale yellow (1-La) to dark green (1-U), thus UV-Vis-NIR spectra of 1-M were recorded at room temperature as 2 mM THF solutions (Figure , see Supporting Information Figures S48-S52 for individual spectra); spectra were collected within 15 minutes of sample preparation at 0 °C thus complex solution equilibria are in operation and some sample decomposition will have occurred (see above), so we restrict our interpretation of these data accordingly. All spectra showed intense charge transfer (CT) absorptions tailing in from the UV region to various extents; for 1-La a maximum was observed at ῦmax = 27,150 cm -1 (ε = 1,760 M -1 cm - 1 ), and for 1-Ce two shoulders were seen at ῦmax = 26,750 cm -1 (ε = 1,010 M -1 cm -1 ) and 23,300 cm -1 (ε = 240 M -1 cm - 1 ); the latter of these absorptions can be assigned to a 4f 1 →5d 1 transition, as previously seen for The spectra of 1-La, 1-Ce and 1-Pr are otherwise featureless, whilst 1-Nd additionally displays a weak set of absorptions in the visible region (ῦmax = 17,250 cm -1 ; ε = 30 M -1 cm -1 ) that we assign as f-f transitions arising from the 4 I9/2 → 4 G5/2 states. In contrast, 1-U exhibits strong absorptions throughout the visible and NIR regions, which gradually decrease in intensity with increasing wavelength (e.g. ῦmax = 20,500 cm -1 , ε = 1,420 M -1 cm -1 ; ῦmax = 8,650 cm -1 , ε = 120 M -1 cm -1 ); similarly strong and broad absorptions were recently observed for [K(18-