Infrared spectra of aqueous solutions of U(VI), Np(VI), Pu(VI), and Am(VI) show conclusively that these ions exist as symmetrical and linear, or nearly linear, XO2++. The spectra of Np(V) and Am(V) show that they are probably XO2+ ions. Force constants and estimated distances are given for the X—O bonds. For the XO2++ series, the X—O force constant is expressed as a parabolic function of atomic number, with the maximum occurring at NpO2++. This is contrary to behavior expected if there were a regular contraction in ionic radii for the series XO2++.
Vibrational data (IR, Raman and inelastic neutron scattering) and
a supporting normal coordinate analysis
for the complex
trans-W(CO)3(PCy3)2(η2-H2)
(1) and its HD and D2 isotopomers are reported.
The vibrational data
and force constants support the well-established
η2-bonding mode for the H2 ligand and provide
unambiguous
assignments for all metal−hydrogen stretching and bending
frequencies. The force constant for the HH stretch,
1.3
mdyn/Å, is less than one-fourth the value in free H2 and
is similar to that for the WH stretch, indicating that
weakening
of the H−H bond and formation of W−H bonds are well along the
reaction coordinate to oxidative addition. The
equilibrium isotope effect (EIE) for the reversible binding of
dihydrogen (H2) and dideuterium (D2) to
1 and 1-d
2
has been calculated from measured vibrational frequencies for
1 and 1-d
2. The
calculated EIE is “inverse”
(1-d
2
binds D2 better than 1 binds H2),
with K
H/K
D = 0.78 at
300 K. The EIE calculated from vibrational frequencies
may
be resolved into a large normal mass and moment of inertia factor (MMI
= 5.77), an inverse vibrational excitation
factor (EXC= 0.67), and an inverse zero-point energy factor (ZPE =
0.20), where EIE = MMI × EXC × ZPE. An
analysis of the zero-point energy components of the EIE shows that the
large decrease in the HH stretching frequency
(force constant) predicts a large normal EIE but that zero-point
energies from five new vibrational modes (which
originate from translational and rotational degrees of freedom from
hydrogen) offset the change in zero-point energy
from the H2(D2) stretch. The
calculated EIE is compared to experimental data obtained for the
binding of H2 or D2
to Cr(CO)3(PCy3)2 over
the temperature range 12−36 °C in THF solution. For the
binding of H2 ΔH = −6.8 ±
0.5
kcal mol-1 and ΔS = −24.7
± 2.0 cal mol-1
deg-1; for D2 ΔH =
−8.6 ± 0.5 kcal/mol and ΔS = −30.0 ±
2.0
cal/(mol deg). The EIE at 22 °C has a value of
K
H/K
D = 0.65 ± 0.15.
Comparison of the equilibrium constants for
displacement of N2 by H2 or D2 in
the complex
W(CO)3(PCy3)2(N2)
in THF yielded a value of
K
H/K
D = 0.70 ±
0.15
at 22 °C.
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