photodissociation spectroscopy of Co+(NH3)n and Ni+(NH3)n: preference for tetrahedral or square-planar coordination
Toshitaka Imamura,a Kazuhiko Ohashi,*b Jun Sasaki,a Kazuya Inoue,a Kazuki Furukawa,a Ken Judai,c Nobuyuki Nishic and Hiroshi Sekiyab
IR spectra of Co+(NH3)n (n = 4–8)without N2 tagging Fig. S1 displays the IR photodissociation spectra of Co+(NH3)n with n = 4–8. Since the D4
value is larger than the IR photon energy, we preferentially detect a warm subset of the n = 4 ions that have internal energies sufficient to assist the elimination of an NH3 molecule following the IR absorption. As a result, the absorption features of Co+(NH3)4 (Fig. S1a) areconsiderably broader than those of Co+(NH3)4·N2 (Fig. 2i).
Co+(NH3)6. The coordination structure of the n = 6 ion is particularly interesting, because Co+Ar6 was found to be anomalously stable,24 and an octahedral coordination was proposed for this ion.22 The experimental IR spectrum of Co+(NH3)6 (Fig. S2a) has a general
resemblance to the n = 5 spectrum. The resemblance makes us suppose that theCo+(NH3)6 ions have 4-coordinated structures that contain Co5IA or Co5IB as a subunit. structures and IR spectra of such (4+2) isomers are given in Fig. S2b–d. The optimized
Co6IA has two
double acceptors in the second shell, whose spectrum is similar to that of Co5IA. Co6IB has two single acceptors, whose spectrum is similar to that of Co5IB. Co6IC has a double and a From our DFT
singleacceptor, whose spectrum is a mixture of those of Co5IA and Co5IB.
calculations, the difference in the energies of these (4+2) structures falls within ≈1 kJ mol-1. The experimental spectrum can be reproduced by a superposition of the theoretical spectra of these (4+2) structures.
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It is worth examining isomerswith other coordination numbers; two examples are depicted in Fig. S2e and f. The DFT calculations locate a (6+0) structure Co6II at a
potential-energy minimum, which has a nearly octahedral arrangement of six N atoms about Co+ with the Co+–N distances of 2.33 Å. The theoretical spectrum of Co6II exhibits two maxima around 3285 and 3395 cm-1. Accordingly, it is not possible to discriminateCo6II However, the coexistence of Co6II may be ruled
from Co6IA on the basis of the IR spectra.
out by reason of its energy, because the DFT calculations predict that Co6II lies 43 kJ mol-1 above Co6IA. Meanwhile, Co6III is a (2+4) isomer with a twofold linear coordination; a
similar structure was found to be dominant for Cu+(NH3)6.20 The transitions of the H-donating NH groups in Co6III arepredicted at 3007 and 3055 cm-1. In the experimental spectrum of Co+(NH3)6, however, there is no appreciable absorption assignable to these transitions of Co6III. the (4+2) structures. In addition, this isomer is computed to be ≈10 kJ mol-1 less stable than
Co+(NH3)7 and Co+(NH3)8.
The spectra of the n = 7 and 8 ions also resemble that of n = 5, The comparison between the experimental andsuggesting a common coordination structure.
theoretical results made in Figs. 4 and S2 demonstrates the preference for the tetrahedral coordination. As we expect that this is also true for n = 7 and 8, we examine a number of
(4+3) and (4+4) isomers having a variety of H-bonding configurations. However, we do not dare to provide the whole set of the data; Fig. S3 shows only a typicalexample for each n, which is compared with the experimental spectrum. Both Co7I and Co8I exhibit the transitions of the H-donating NH groups in the 3100–3300 cm-1 region. These transitions
are likely to contribute to the red-shifted features observed in the experimental spectra, although it is probable that there coexist other isomers that have a similar tetrahedral first shell in common but...