.delta. .fwdarw. .delta.* Transition energies as a function of .delta.-bond strength: an extrapolative assessment of the ground-state electron correlation energy
“…It is clear that for the D 2 h and the D 2 d molecules, in both of which the metal−ligand bonds are eclipsed, the absorption bands are at about 570 nm (17.5 kcm -1 ), whereas for the compound in which the molecule has a D 2 conformation (N−Mo−Mo−N torsion angle of 19.8°) the band is at 645 nm (15.5 kcm -1 ). This is in reasonable agreement with expectation based on comparison with the known behavior of Mo 2 Cl 4 (PR 3 ) 4 compounds 2 Visible spectra of crystalline Mo 2 Cl 4 (4-Bu t -py) 4 compounds suspended in Nujol.…”
The relative stabilities of rotation conformational isomers of Mo2Cl4(R-py)4 molecules in solution
have been studied. R-py represents a substituted pyridine, 4-tert-butylpyridine, 4-picoline, or 3,5-lutidine, and
the MoCl2(R-py)2 portions of each molecule have the trans configuration. In previously reported crystal
structures, these molecules have been found with several different conformations: D
2
h
, D
2
d
(both eclipsed),
and D
2 (partly staggered). From the visible spectra of these solids, it is found that the D
2
h
and D
2
d
conformations
give rise to a δ→δ* band at ∼570 nm, while a D
2 conformation with a twist angle of ∼20° from D
2
h
has a
band at ∼645 nm. For solutions in CH2Cl2 we find both bands, but the intensity ratio is strongly dependent
on temperature. In addition the intensity ratio varies over a range of 100 in THF/CH2Cl2 mixtures. Calculations
by the DFT method provide an estimate of the energy variation as a function of internal rotation angle from
the D
2
h
through D
2 to D
2
d
conformations. The major conclusions are: (1) the D
2
h
conformation, although
frequently found in crystals, is not stable in solution or the gas phase; (2) the relative stabilities of the D
2
d
and
D
2 conformations vary greatly with solvent and temperature; (3) absorption bands at ∼25 000 cm-1, 20,000
cm-1, and 15,000−18,000 cm-1 can be assigned to δ→d
x
2
-
y
2
, π→δ*, and δ→δ* transitions, respectively.
“…It is clear that for the D 2 h and the D 2 d molecules, in both of which the metal−ligand bonds are eclipsed, the absorption bands are at about 570 nm (17.5 kcm -1 ), whereas for the compound in which the molecule has a D 2 conformation (N−Mo−Mo−N torsion angle of 19.8°) the band is at 645 nm (15.5 kcm -1 ). This is in reasonable agreement with expectation based on comparison with the known behavior of Mo 2 Cl 4 (PR 3 ) 4 compounds 2 Visible spectra of crystalline Mo 2 Cl 4 (4-Bu t -py) 4 compounds suspended in Nujol.…”
The relative stabilities of rotation conformational isomers of Mo2Cl4(R-py)4 molecules in solution
have been studied. R-py represents a substituted pyridine, 4-tert-butylpyridine, 4-picoline, or 3,5-lutidine, and
the MoCl2(R-py)2 portions of each molecule have the trans configuration. In previously reported crystal
structures, these molecules have been found with several different conformations: D
2
h
, D
2
d
(both eclipsed),
and D
2 (partly staggered). From the visible spectra of these solids, it is found that the D
2
h
and D
2
d
conformations
give rise to a δ→δ* band at ∼570 nm, while a D
2 conformation with a twist angle of ∼20° from D
2
h
has a
band at ∼645 nm. For solutions in CH2Cl2 we find both bands, but the intensity ratio is strongly dependent
on temperature. In addition the intensity ratio varies over a range of 100 in THF/CH2Cl2 mixtures. Calculations
by the DFT method provide an estimate of the energy variation as a function of internal rotation angle from
the D
2
h
through D
2 to D
2
d
conformations. The major conclusions are: (1) the D
2
h
conformation, although
frequently found in crystals, is not stable in solution or the gas phase; (2) the relative stabilities of the D
2
d
and
D
2 conformations vary greatly with solvent and temperature; (3) absorption bands at ∼25 000 cm-1, 20,000
cm-1, and 15,000−18,000 cm-1 can be assigned to δ→d
x
2
-
y
2
, π→δ*, and δ→δ* transitions, respectively.
“…The most important result of these studies in the present context was that there is a relationship between the twist angle and the energy of the δ → δ* absorption band, as shown in Figure . We see here that the extrapolated δ−δ* transition energy at χ = 45° (cos 2χ = 1) is not zero even though the δ−δ overlap is zero.…”
Section: A Brief Chemical Backgroundmentioning
confidence: 62%
“… 10 Actual experimental state energies vs cos 2χ for the Mo 2 X 4 (P−P) 2 molecules. Data from refs , , , and 22.…”
Section: Energy Measurements Of the δ Bond Manifoldmentioning
It is shown that the delta bond, as found particularly in the Re(2)(6+) and Mo(2)(4+) cores of hundreds of compounds, provides a paradigm for the behavior of two-electron bonds of all types. By control of the angle of twist around the M-M axis, the strength of the bond can be systematically varied. By means of conventional electronic spectroscopy, nuclear magnetic resonance spectroscopy, and two-photon excitation spectroscopy, the entire picture of the manifold of four states for two electrons bonding two atoms, as first described by Coulson and Fischer in 1949, has been confirmed.
“…As explained in part 1 of this series, Mo 2 Br 4 (dmpe) 2 , with only 29% δ character properly belongs with the triple bonds. It was found 1 that steric distortion of planar bridging ligands across the dimetal center prevents effective δ interaction to the point where it no longer contributes to the bonding.…”
Through molecular modeling of the available structures containing dimolybdenum bonds of orders 3.5 and 3,
solution pairs of harmonic force constant (k
r) and bond length (r
o) have been obtained as (3.63 mdyne/Å, 2.07 Å)
and (3.37 mdyne/Å, 2.10 Å), respectively. Together with the known solution pair for MoMo, this sufficed to
establish a general relationship between k
r, r
o and bond order, N, as N = 0.9537k
r = 131.8r
o
-5. Using this
relationship as a sampling curve, we extended the analysis to include Mo2 bonds of orders 2 and 1. The relationship
was accordingly shown to apply to the entire {N, k
r} space.
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