Multiple Bonds Between Metal Atoms / 16-Physical, Spectroscopic and Theoretical Results

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In 1988 the results of an MCD measurement236 showed that the sign of the MCD for band A agreed with expectation for an upper A2u (μ(Rh2)) orbital but was the reverse of that expected for an upper B2u (μ*(Rh–O)) orbital, thus supporting the original assignment, which is now accepted.

The assignment of band B, also xy-polarized and showing no resolved vibrational structure,233,235 is at present still uncertain. It has been assigned as a /(Rh–O)Αμ* (Rh–O) transition.235 There are also strong absorption bands in the near UV (40,000-45,000 cm-1) for which a μ(Rh2)Αμ*(Rh2) assignment has been proposed.163,235

Pt2(O2CR)4L2, Pt2(O2SO2)4L2 and Pt2(O2P(O)OH)4L2.

While these have the same type of ground state electron configuration, μ2/4β2β*2/*4, as their Rh24+ analogs, there is a great deal more mixing of metal and ligand orbitals. Spectra are, accordingly, more complex and difficult to assign,237-240 and the details are beyond the scope of this discussion. For any given set of bridging ligands, the axial ligands may be varied (e.g., H2O, Cl-, Br-, NCS-) and such variations result in large changes in the spectra. There is no doubt that essentially all observed bands have considerable LMCT character. It should be noted that the MCD results in ref 238 appear to refute some of the assignments proposed in ref 237. The assignments in ref 239 and ref 240 appear to be the most reliable.

[Ru2(O2CR)4]0,+ and related compounds.

Many spectroscopic observations have been mentioned in Chapter 9. For Ru25+ species with a quartet ground state derived from a μ2/4β2 (β*/*)3 configuration, a βΑβ* type transition should occur effectively as a one-electron transition and thus at about the calculated energy. This is the case. The calculations167 place it at about 8800 cm-1 and polarized crystal spectra 241-243, confirm this assignment for a band with an origin near 9000 cm-1 and displaying a progression in ι'(Ru–Ru). A very weak absorption at c. 7000 cm-1 has been assigned to a /*Αβ* transition.243,244

The most prominent feature in the spectra of all the molecules is an intense band around 21,000 cm−1 and all the evidence243,245 as well as theory167 favor assigning this to the 6euΑ6eg transition, where 6eu is essentially a /-orbital that shares both oxygen and metal / character and 6eg is the /*(Ru2) orbital.

Os2(O2CR)4Cl2 molecules.

These have been discussed in Chapter 10. Like their ruthenium homologs, they have μ2/4β2 (β*/*)2 ground states.246 Their spectra are complex, but plausible assignments have been made. A z-polarized βΑβ* transition occurs at c. 12,000 cm-1 and displays a progression in the excited state ι(Os–Os) vibration (220 cm-1).

[Os2X8] 2-.

The [Os2X8]2- ions (to which there are no ruthenium homologs) have also been discussed in Chapter 10. They have D4d symmetry and μ2/4β2 β*2 ground state configurations. The absorption spectra for X = Cl, Br, and I have been reported.247 All of them display a plethora of bands between 250 and 750 nm of which only the lowest in each case has been assigned, namely, to a βΑ/* excitation. The lengths of the progressions and the considerable reductions in frequency from the ground state values (c. 90 cm-1) are consistent with this assignment.

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16.4.5 CD and ORD spectra

Compounds containing M–M quadruple bonds can be chiral and when they are they display CD and ORD effects (optical activity). In practice only the CD effects have been studied. As is true of optically active compounds in general, there are two major categories:

1.The inherently chiral chromophore.

2.The achiral chromophore in a chiral environment.

Examples of category A are provided by molecules in which one to four chiral ligands bridge the metal atoms and impose a twist about the M–M bond so that it is no longer in an eclipsed state. The commonest, but not the only, examples are the `-Mo2X4(PP)2 species, in which PP represents a 1,2-diphosphinoethane ligand. Because the chromophore itself, that is, the M–M quadruple bond and its coordinated atoms, has a helical conformation, it is inherently chiral. Examples of category B are provided by carboxylato-bridged species, M2(O2CR)4L2 in which the eclipsed conformation exists but either R or L is chiral, or by the chelated (_) isomers of M2X4(diamine)2, in which the diamine is chiral. These two cases require rather different theoretical analyses, case A being much more straightforward, and we shall now discuss them separately, beginning with type A.

The prototype compound to illustrate case A, the inherently chiral chromophore, is Ρ-[Mo2Cl4(S,S-dppb)2], where the diphosphine ligand is (S,S)-Ph2PCHMeCHMePPh2. Conformational analysis predicts and crystallography confirms248,249 that with the (S,S)-ligand the Ρ sense of rotation about the Mo–Mo bond should be induced. This molecule is shown in Fig. 16.35. As viewed straight down the Mo–Mo axis, the left or counterclockwise twist (by c. 23˚) is clearly evident. Fig 16.35 also shows the CD spectrum of the same molecule and it can be seen that there are two very prominent features: a negative CD band corresponding to the β2Αββ* absorption (c. 13,500 cm-1) and a positive CD band at c. 21,300 cm-1. Similar CD spectra are observed for many other similar molecules. 250-252 All such results can be understood in terms of the following straightforward analysis.248,249,251

Fig. 16.35. The `-Mo2Cl4(S,S-dppb)2 molecule viewed down the Mo–Mo axis (right) and its CD spectrum (left).

For the β2Αββ* transition, whose assignment is securely established, the transient charge distribution during the transition is shown diagrammatically in Fig. 16.36. It can be seen that based on this diagram we can state that the βΑβ* transition has a movement of charge both

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along and around the Mo–Mo bond. This means that it is both electric dipole allowed and (due to the rotation) magnetic dipole allowed. The combination of these two qualities makes it CD active. Moreover, it is possible, as also shown in Fig. 16.36, to infer the sign of the CD band because this is a consequence of the direction of charge rotation. We take the dipole direction to be given by the + Α − direction. We then take the charge rotation in the same sense, and assign a vector to the rotation according to the right hand rule: if fingers point in the direction of rotation, the thumb points in the vector direction. We can thus see in Fig. 16.36 (a) that for the Ρ molecule the electric and magnetic vectors point in opposite directions (down and up, respectively). This means that the CD band should be negative for the β2Αββ* transition of a Ρ-M2X4(PP)2 type molecule, as observed for Ρ-Mo2Cl4(S, S-dppb)2.

Fig. 16.36. Diagrams of the transient charge distributions for the βΑβ* transition in twisted Mo2X4(LL)2 molecules with twist angle ε (a) in the range 0 to -45° and (b) in the range -45 to -90°. Note that the two ranges, though in the same direction geometrically, give transient charge distributions of opposite rotational sense.

This analysis can be generalized into a sign rule as shown in Fig. 16.37. This sign rule has the following important feature. For a rotation of > 45°, the CD sign again changes (see Fig. 16.36 (b)) and it therefore turns out that for rotations of ± ε the sign of the CD will be the same as for rotations ± ε. The first actual test of this complete relationship was provided250 by the compound Mo2Cl4(S, S-chiraphos)2, in which the mean P–Mo–Mo–P torsion angle is c. -80°, that is, into the region where the CD band for the βΑβ* transition should be positive, and it is.

Fig. 16.37. The sign rule for the CD of the βΑβ* transition. The sign of the CD refers to the sector in which the rear set of ligand atoms is found.

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As would be expected, the CD spectrum of the [Re2Cl4(S,S-dppb)2]2+ ion (which has a μ2/4β2 configuration) conforms to the same sign rule as the isostructural and isoelectronic molybdenum compounds.253 The Mo2Cl4(LL)2 (LL = (R)-H2NCH(CH3)CH2NH2) molecule also appears to be of type A, and it too, follows the octant sign rule.254

In the type A compounds we have just discussed, the CD band at around 22,000 cm-1 is opposite in sign to the CD band for the β2Αββ* transition. This is naturally, and without exception, explained by assigning the 22,000 cm-1 band to the transition βxyΑβx2-y2, for which it is easily shown251 that an octant type sign rule also applies, but rotated 45° from the one we have derived for the βΑβ* transition. Thus, by the correct use of CD spectra one can conclusively refute the suggestion255 that the lowest-energy band in the spectra of quadruply-bonded species should be assigned to a βxyΑβx2-y2 transition rather than to a βΑβ* transition.

We now turn to compounds of class B, in which there is no internal twist to make the M–M bond inherently chiral, but instead an essentially eclipsed M2X8 core within a set of ligands some or all of which are chiral. The earliest attempt256 to deal with such a compound was concerned with rhodium compounds of the type Rh2(O2CR)4L2, where R = CPh(OH)H and CPh(OMe)H. As explained fully at the time256 this situation is more difficult to analyze because within the chromophore no transition is both electrically and magnetically allowed. Hence, a perturbation method whereby some magnetic component is mixed into a nominally dipoleallowed transition, or vice versa must be employed. The details are too complex to be spelled out here and the original papers dealing with the dirhodium compounds236,256 and others that belong to the same class257,258 should be consulted.

A different type of class B compound was more recently examined, namely [Mo2(O2CCF3)2- (S,S-dach)2(CH3CN)2](BF4)2 in which the S,S-dach (dach = 1,2-diaminocyclohexane) ligands are chelated, one to each Mo atom. The R,R-dach enantiomer was also characterized.259 In an earlier report in which structure was not determined, it was assumed that the dach ligands were bridging and the CD spectrum was treated as a class A case.260

A final point of importance has to do with the employment of M2n+ complexes as tools for studying the absolute chiralities of colorless organic compounds in solution. Organic chemists have long been interested in the idea of adding some metal-containing species with electronic absorption in the visible region to a solution containing the organic compound of interest so that when the former forms a complex with the latter, it will acquire a CD spectrum in the conveniently observed visible region. No really practical and general way to do this was found until recently. Snatzke and co-workers261 made a number of attempts to employ Mo2(O2CCH3)4, whose β2Αββ* transition is conveniently placed (c. 450 nm) but without finding a fully satisfactory method. However, it has recently been found that Rh2(O2CCF3)4 can bind essentially every type of organic molecule at its axial positions,262 including even olefins,263 and then display CD effects whose signs can be related to the absolute configuration of the attached organic molecule. It appears that Rh2(O2CCF3)4 may turn out to be the long-sought general reagent for absolute chirality determinations.

16.4.6 Excited state distortions inferred from vibronic structure

It is well known that in principle it is possible to calculate, at least approximately, structural changes in a molecule upon electronic excitation or ionization from the vibrational patterns observed in the electronic absorption band or PES ionization band. This has been done for several Mo2(O2CR)4 compounds, Mo2[(CH2)2PMe2]4, and Rh2(O2CMe)4L2. Extensive work has been done on the Mo compounds, where progressions in the ι'(Mo–Mo) vibration are employed and the process is commonly referred to as Franck-Condon analysis. The first such result was for Mo2(O2CCH3)4 where the progression in ι'(Mo-Mo) on the β2Αββ* excitation

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(then thought to be a βΑ/* excitation) was used.212 The Mo–Mo distance was estimated to be c 0.1 Å longer in the excited state. From the analogous vibronic data for Mo2(O2CCF3)4 and Mo2[(CH2)2PMe2]4 estimates of 0.045264 and 0.09 Å,220 respectively, have been made, while for the [Mo2X6(H2O)2]2- ions (X = Cl, Br) the derived values are 0.12-0.13 Å.265 A combined study of resonance Raman and electronic absorption spectra of Mo2X4(PMe3)4 molecules has also led to a value of 0.10 Å for the Mo–Mo bond length increase in the singlet state of the μ2/4ββ* configuration.266

A related but more sophisticated approach which employs both a Franck-Condon analysis of the β2Αββ* absorption band and the intensities of resonance Raman overtones for the ι'M–M vibration is called the sum-over-states method. It has been applied to the [Re2Br8]2-, [Re2I8]2- and [Mo2Cl8]4- ions.267 The results are similar to those previously obtained, namely an increase of 0.08 Å in the Re–Re bond distances and 0.15 for Mo–Mo, on going from the 1A1g ground state to the 1A2u excited state.

From the vibration progression in the β ionization of Mo2(O2CCH3)4 (see Fig. 16.44) it was estimated268 that the Mo–Mo distance in the [Mo2(O2CCH3)4]+ ion is 0.13-0.18 Å longer than that in the neutral molecule. This result must be considered surprising because the ionization process abolishes only half of the β-bond whereas the β2Αββ* transition abolishes all of it. It was proposed268 that the increase in oxidation state of the Mo atoms upon ionization also makes a substantial contribution to bond lengthening, but this would still leave some inconsistency between the two types of result. This inconsistency prompted a reanalysis269 of the ionization results, from which it was concluded that the change in distance was probably 0.11 Å.

These spectroscopic results may be compared with some X-ray crystallographic results which were summarized in Section 16.1.1. In the series [Mo2(SO4)4]4-, [Mo2(SO4)4]3-, [Mo2(HPO4)4]2- where at each step there is loss of one β-electron and a one-unit increase in oxidation state, the Mo–Mo bond length increases are each about 0.06 Å. This is reasonably consistent with the recalculated increase on photoionization of Mo2(O2CCH3)4, 0.11 Å. On the other hand, the structures of Mo2(DTolF)4 and Mo2(DTolF)4+ show only a 0.037 Å increase on ionization.177

For the Rh2(O2CCH3)4L2, L = Ph3P or Ph3As, molecules, Franck-Condon analysis270,271 of the progressions seen in a band believed to be due to a /*Αμ* transition, have led to ¨(Rh–Rh) 5 0.045 Å and also ¨(Rh–O) 5 0.038 Å. The μ* state is believed to be one in which the excited electron is in an orbital that is primarily Rh–Rh antibonding, but some μ* Rh–O character can also not be excluded.

For the most commonly observed βΑβ* type transition, namely from a 1A1g (β2) ground state to a 1A2u (ββ*) excited state the molecule passes on a very fast time scale (c. 10-16 sec) from an electronic structure in which there is a β-bond strong enough to maintain an internal rotation angle of c. 0° to an electronic structure in which no β-bond exists and the most stable structure would be one in which the preferred internal rotation angle is 45°. In other words, the spectroscopically observed 1A2u state, which is responsible for the observed vibrational structure of the absorption band, is an unrelaxed state for the molecule having a μ2/4ββ* electron configuration. The relaxed configuration, as in molecules with μ2/4 or μ2/4β2β*2 configurations, should have a torsion angle of 45° (D4d symmetry instead of D4h).

The existence of such relaxed μ2/4ββ* molecules has been demonstrated in two ways. One approach is to use time-resolved resonance Raman (TR3) spectroscopy. In this way the excited state geometry can be probed.272 For solids containing [Re2X8]2- (X = Cl, Br) ions it is seen that until the eclipsed 1A2u excited state decays back to the ground state, it retains its D4h structure because it is constrained by crystal packing forces. In solution, however, the conformation changes within nanoseconds to D4d as evidenced by the Raman spectrum. A second type of experiment entailing the study of emission spectra will be discussed in the next section.

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16.4.7 Emission spectra and photochemistry

Emission spectra.

The emission spectra of the [Mo2Cl8]4-, [Re2Cl8]2-, and [Re2Br8]2- ions and especially the Mo2X4(PR3)4 compounds have been studied in detail. The earliest reported observations of the [M2X8]n- ions were as follows. Excitation of solid compounds containing [Re2Cl8]2- and [Re2Br8]2- ions at 650 nm or [Mo2Cl8]4- at 540 nm, at 1.3 K, generated broad emission bands at frequencies below those of the respective β2Αββ* absorption bands.273 The two most important features of these results were that:

1.the absorption and emission spectra were not mirror images, and

2.the absorption and emission envelopes did not overlap at the frequency of the 0-0 transition in the absorption band.

It was therefore concluded that these emissions could not be attributed to simple radiative decay of the 1A2u (ββ*) state. Instead, it was suggested, the emission is from one of the spin-orbit components of the 3A2u state arising from the same configuration. This work was followed up by a study274 of the emission behavior of Mo2Cl4(PBun3)4 which gave the results shown in Fig. 16.38. Here the absorption and emission envelopes are essentially mirror images and overlap at the 0-0 band; this is clearly a simple case of prompt emission from the singlet excited state. The obvious question was, then, why this case is so different from that of [Mo2Cl8]4- and the [Re2X8]2- ions. There are also further details concerning the emission behavior of the [Re2Cl8]2- ion that are not easily reconciled with the previously proposed 3A2uΑ1A1g emission process.274,275

Fig. 16.38. Absorption (left) and emission (right) spectra of Mo2Cl4(PBun3)4 at 80 K in a 2-methylpentane glass.

It was then proposed274,275 that the foregoing observations can be reconciled by recognizing that in the ββ* excited state the eclipsed rotational conformation is no longer stable relative to the staggered one (the β-bond has been abolished). From the [M2X8]n- ions, then, the emitting state is one in which a rotation to (or towards) the staggered conformation has occurred. That being the case, no mirror image relationship to the absorption spectrum is to be expected. In Mo2Cl4(NBun3)4 such a rotation is prevented by the tight interlocking of the large and small ligands and the ground state and excited state structures are so similar that the mirror image relationship is seen.

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Further work276-278 has been done on the three Mo2X4(PMe3)4 compounds with X = Cl, Br, and I, which also show emission spectra indicative of close geometrical similarity of the ground and excited states, but to different degrees, with the iodide providing the most and the chloride the least perfect mirror images. Only recently279 has emission from the 3A2u state been shown to occur, namely in Re2(DAniF)4Cl2.

It was noted in Section 16.4.6 that a TR3 study of the Re2Cl82- ion had shown that while it rapidly internally rotates to a D4d structure in solution (as would be expected), it cannot and does not do so in a crystalline environment. This, along with other subsequent work,280 in which it was shown that an earlier report on solid (NBun4)2[Re2Cl8] was incorrect, puts an end to the need for strained rationalizations.274,275 Solid (NBun4)2[Re2Cl8] emits from the 1A2u state of the D4h anion.

In an important study281 employing picosecond excitation followed by transient absorption spectroscopy, it was found that in fluid solution at room temperature, both [Re2Cl8]2- and [Mo2Cl8]4- give, in less than 20 picoseconds, a transient that is reasonably attributable to the twisted, singlet excited state. This same study produced other interesting information about transient excited states in quadruply bonded species. This accords with the TR3 study which showed272 that after a few nanoseconds [Re2Cl8]2- in its 1A2u (ββ* ) excited state has a staggered conformation when it is in solution.

It has also been shown that the emission of Mo2Cl4(PMe3)4 can be electrogenerated.282 This is done by pulsing the applied potential from a value more positive than that required for oxidation to one more negative than that for reduction, thus generating both cation and anion radicals in close proximity. Because the energy released on recombination exceeds that required for an excitation to the ββ* state (the [Mo2Cl4(PMe3)4]* species, which emits) we have the following reaction sequence:

[Mo2Cl4(PMe3)4]- + [Mo2Cl4(PMe3)4]+ Α Mo2Cl4(PMe3)4 + [Mo2Cl4(PMe3)4]*

[Mo2Cl4(PMe3)4]* Α Mo2Cl4(PMe3)4 + hι

Only a few other observations of emission from excited states have been reported. For Mo2(O2CCF3)4 structured emission has been observed at 1.3 K with an origin 1800 cm-1 below that of the absorption band, which was then assigned to a βΑ/* transition,212 and the emission to the reverse tripletΑsinglet transition. Since we now know that the absorption band is the β2Αββ* absorption, the emission should be reassigned also, to the 3A2uΑ1A1g transition. The long life of the excited molecule (2 ms as compared to an estimated 2 µs for a 1A2u state) as well as the 1800 cm-1 separation of the origins are the basis for designating the emitting state

as 3A2u rather than 1A2u .

The compounds Mo2(mhp)4, Mo2(chp)4 and W2(mhp)4 have been observed to emit upon excitation into the β2Αββ* absorption band.283 All show vibrational structure (at 15 K) and in the case of Mo2(mhp)4 it is highly resolved. It was not possible, however, to make a firm assignment of the emitting state.

Photochemistry.

The photochemistry of [Re2Cl8]2-, via its singlet ββ* excited state has been developed in an interesting way by Nocera and Gray. They first showed284 that the luminescence of this species, hereafter [Re2Cl8]2-*, is quenched by both electron acceptors, which remove the β*- electron to give [Re2Cl8]-, and electron donors (aromatic amines), which add a β-electron to give [Re2Cl8]3- as a strongly associated ion pair, (amine+) ([Re2Cl8]3-). Back reactions in both cases are extremely fast. A diagram showing the energetic relationships of the four pertinent

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Re2Cl8 species was deduced and is as shown below, where the units are eV or V versus SCE in CH3CN solution:

These workers then showed285 that the uninteresting thermal back reaction of [Re2Cl8]- and the quencher, Q, can be obviated by the presence of Cl- ion. In this case, one of two things will happen, depending on whether Q is a relatively weak oxidizing agent, or a stronger one, as the following two reactions show:

[Re2Cl8]2-* + Q + Cl- Α Q- + [Re2Cl9]2-

[Re2Cl8]2-* + 2Q + Cl- Α 2Q- + [Re2Cl9]-

Irradiation of [Mo2(SO4)4]4- in aqueous H2SO4 at 254 nm causes the following reaction223,286 (quantum yield, 17%):

2[Mo (SO ) ]4- + 2H O+ hι 2[Mo (SO ) ]3- + 2H O + H

2 4 4 3 Α 2 4 4 2 2

Generation of hydrogen can also be caused by irradiation of other Mo24+ species.286 Thus Mo24+ (aq) in CF3SO3H undergoes the following reaction (in low quantum yield, 3.5 per cent):

Mo24+ (aq) + 2H2O Α [Mo2(µ-OH)2]4+ (aq) + H2

More recently it has been shown287 that [Mo2(HPO4)4]4- displays similar, but even more elaborate photochemistry. Here there is a series of three species, the 4-, 3-, and 2- ions, and irradiating either of the first two leads to the 2- ion, with evolution of ½H2 at each step. From a study of the wavelength profile for photoactivity and the absorption spectra, it was concluded that the photoactive state is in each step one that is produced by a /Α/* excitation. The highly reducing /Α/* excited state then directly reduces H+ to H, and the H atoms rapidly combine to form H2.

When [Mo2X8]4-, X = Cl, Br, are irradiated in aqueous HX solution,286 [Mo2X8H]- is first formed and then undergoes decomposition to give H2 and [Mo2(µ-OH)2]4+. The glycine complex, [Mo2(O2CCH2NH3)4]4+, does not react in this way.286 Given the high energy of the photons used (254 nm) in these reactions, it is not the singlet ββ* state that is generated, but some more highly excited one, perhaps again one resulting from a /Α/* excitation.

In contrast to the systems just discussed where a /Α/* or other high-energy excited state is responsible for the photochemistry, the Mo2[O2P(OPh)2]4 molecule has allowed for the study of photochemistry in nonaqueous media where the β2Αββ* excitation is responsible for photoactivation.288 Using 510 nm light (the β2Αββ* transition causes absorption maximizing at 515 nm) the following reaction can be carried out with a quantum yield of 4%:

2Mo2[O2P(OPh)2]4 + ClCH2CH2Cl Α 2 Mo2[O2P(OPh)2]4Cl + C2H4

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This overall stoichiometry is consistent with either of two pathways, once the activated Mo2 species, Mo24+*, has reacted with ClCH2CH2Cl to give MoIIMoIIICl + ClCH2CH2. If these two are held in a tight solvent cage, further reaction to give C2H4 and ClMoIIIMoIIICl probably ensues. There is then a comproportionation of ClMoIIIMoIIICl with MoIIMoII to give MoIIMoIIICl. On the other hand a free ClCH2CH2 radical may react with MoIIMoII to give MoIIMoIIICl

and C2H4.

In all of the photochemistry of quadruply-bonded dimetal compounds so far discussed, only one-electron transfers occurred; overall two-electron redox reactions occurred stepwise. One of the goals in investigations of the photochemistry of these binuclear systems was to see if any genuine two-electron process could be discovered.289 In the photochemical reaction of W2Cl4(dppm)2 with CH3I, this goal has been reached.290 While thermal additions to quadru- ply-bonded molecules, which proceed by radical processes, give scrambling of ligands, the photoaddition of CH3I to W2Cl4(dppm)2 gives a single pure product. It is believed289 that the photoactivation occurs through a βΑ/* or a /Αβ* excitation (or both, since the two excited states are accidently almost degenerate).

A few other results of a photochemical nature have been reported. Irradiation of a solution of [Re2Cl8]2- in acetonitrile291 with a 1000-watt Hg-Xe lamp equipped with a pyrex filter causes cleavage of the dinuclear species and allows isolation of ReCl3(CH3CN)3 as well as a small amount of [ReCl4(CH3CN)2]-. Further study292 left the detailed mechanism still in doubt.

While it is not, strictly speaking, photochemistry, since no net chemical change occurs, flash photolysis of Mo2(O2CCF3)4 in acetonitrile or benzene at 337 nm causes bleaching, followed by the reappearance of ground state absorption on a microsecond time scale; the recovery follows first-order kinetics, with a half-life of 33 µs in benzene.293 The principal species present at the end of a 10 ns flash was postulated to be a triplet state derived from the μ2/4ββ* configuration (incorrectly assigned in the paper because of the confusion generally prevailing at the time concerning the absorption bands at c. 23.0 x 103 cm-1 for Mo2(O2CR)4 compounds as a class). Some speculative discussion was presented concerning possible intermediate adducts with CH3CN solvent.

An odd observation294 of uncertain significance is that four Rh2(O2CCH3)4L2 (L = CH3OH, THF, PPh3, py) compounds are excited by “visible light” to a transient excited state of 3-5 µs lifetime which has an absorption band at c. 760 nm. No suggestion was made as to what this transient is.

Effects of high pressure.

It is well known that the compression of liquid solutions and molecular solids entails mainly decreasing the intermolecular distances where the softest (van der Waals) resistance is encountered. Nevertheless, at sufficiently high (50-150 kbar) pressures molecular shapes and dimensions are also affected and there are consequences seen spectroscopically.377,421-425 Interpretation of the observations is somewhat speculative and there are differences of opinion. Increasing pressure causes increases in ι(M–M) and decreases in the energy of the βΑβ* type transition in, for example Re2Cl82-. Both have been, reasonably, attributed to concomitant twisting, which lessens the β bond strength, and shortening of the Re–Re distance. It is easier to compress the Re–Re bond when the conformation is twisted away from the eclipsed state. For Mo2Cl4(PMe3)4, it has been proposed that the smaller spectroscopic changes result from compression alone.

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16.5 Photoelectron Spectra

Photoelectron spectra (PES) provide the most direct and least equivocal experimental information about valence electrons in molecules. In this context we are referring to the use of UV light to photodetach valence shell electrons. Inner shell electron spectroscopy, denoted XPS, will be mentioned in Section 16.7.2.

16.5.1 Paddlewheel molecules

Since the entire field to which thus book is devoted commenced with the Re2Cl82- ion, we turn first to the PES of that species. The experimental methodology for PES measurements on gaseous ions, which is novel, was applied to Re2Cl82- in 2000 and gave the results295 shown in Fig. 16.39. It is self-evident that these data confirm unequivocally the original proposal of a quadruple bond in the Re2Cl82− ion (see Section 1.2.2). Distinct β, / and μ ionizations, preceding a plethora of peaks from other ionizations, exactly as expected from theory, are clearly to be seen.

Fig. 16.39. The photoelectron spectrum of [Re2Cl8]2- showing the assignment of the features to the molecular orbitals.

We turn now to a historical account of PES studies. The easy volatility and relative simplicity of the group 6 M2(O2CCH3)4 molecules made them early subjects of study,296-302 although some parts of the interpretations accepted today differ from those first proposed.304 The He(I) PE spectra of these molecules304 are shown in Fig. 16.40. There is a marked difference between the lower-energy region for the chromium compound and the other two. As shown in Fig. 16.41, the first broad band in Cr2(O2CCH3)4 can be deconvoluted into three overlapping bands in an approximately 1:2:1 intensity ratio. It is generally believed that these correspond to the β, / and μ ionizations, in increasing order of energy.

The Mo2(O2CCH3)4 and W2(O2CCH3)4 spectra each begin with a distinct weak band that can be assigned to the β ionization. The spectra of the Mo and W carboxylates differ in their next highest bands, the Mo compound showing only a single (although slightly unsymmetrical) band while the W compound has two bands, the one at the higher energy being very sharp. W2(O2CCF3)4 has been shown to display this same pattern.305 According to an early assignment of the molybdenum spectrum, the single observed band, at c. 9 eV. corresponds to the / ionization only, with the μ ionization lying at least 1.5 eV higher and thus buried in the first