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The electronic spectra of Co+(H2O), Co+(HOD), and Co+(D2O) havebeen measuredfrom 13 500 to 18 400 cm–1using photodissociation spectroscopy. Transitionsto four excited electronic states with vibrational and partially resolvedrotational structure are observed. Each electronic transition hasan extended progression in the metal–ligand stretch, v3, and the absolute vibrational quantum numbering is assignedby comparing isotopic shifts between Co+(H216O) and Co+(H218O). For thelow-lying excited electronic states, the first observed transitionis to v3′ = 1. This allows the Co+–(H2O) binding energy to be determined as D0(0 K)(Co+–H2O) = 13730 ±90 cm–1(164.2 ± 1.1 kJ/mol). The photodissociationspectrum shows a well-resolved Kaband structure due to rotation about the Co–O axis.This permits determination of the spin rotation constants ϵaa″= −6 cm–1and ϵaa′= 4cm–1. However, the Karotational structure depends on v3′.These perturbations in the spectrum make the rotational constantsunreliable. From the nuclear spin statistics of the rotational structure,the ground state is assigned as 3B1. The electronictransitions observed are from the Co+(H2O) groundstate, which correlates to the cobalt ion’s 3F,3d8ground state, to excited states which correlate tothe 3F, 3d74s and 3P, 3d8excited states of Co+. These excited states of Co+interact less strongly with water than the ground state.As a result, the excited states are less tightly bound and have longermetal–ligand bonds. Calculations at the CCSD(T)/aug-cc-pVTZlevel also predict that binding to Co+increases the H–O–Hangle in water from 104.1° to 106.8°, as the metal removeselectron density from the oxygen lone pairs. The O–H stretchingfrequencies of the ground electronic state of Co+(H2O) and Co+(HOD) have been measured by combiningIR excitation with visible photodissociation in a double resonanceexperiment. In Co+(H2O) the O–H symmetricstretch is ν1″ = 3609.7 ± 1 cm–1. The antisymmetric stretch is ν5″ = 3679.5± 2 cm–1. These values are 47 and 76 cm–1, respectively, lower than those in bare H2O. In Co+(HOD) the O–H stretch is observed at 3650cm–1, a red shift of 57 cm–1relativeto bare HOD. [ABSTRACT FROM AUTHOR]