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

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Fig. 16.51. An example of excitation profiles in RR spectra. Upper curve is the electronic absorption spectrum of the [Re2F8]2- ion, featuring the β2Αββ* transition. Below are plots of Raman line intensities versus frequency for the ι(Re–Re) line and its first two overtones.

An instructive example337 of the danger of superficial interpretation is afforded by the [Mo2(CN)8]4- ion, where isotopomers containing (nearly) all 12C and (nearly) all 13C can be compared. The Raman band at 411 cm-1 in the 12C ion which is resonance-enhanced by excitation in the βΑβ* band (at c. 600 nm) would, loosely speaking, be called the ι(Mo–Mo) band. Yet, with data from both isotopomers, this band (which shifts only a little to 406 cm−1, in the 13C isotopomer) is found to be far from a pure Mo–Mo stretch. In fact, the normal coordinate for this vibration has only 50-60% ι(Mo–Mo) character, and 30-40% Mo–C–N wagging character.

Similarly, in Mo2(CCH)4(PMe3)4 a normal coordinate analysis based on data for isotopomers showed that the “ι(Mo–Mo)” Raman band has only about 54% ι(Mo–Mo) character combined with 18% ι(Mo–C), 12% β(Mo–Mo–C) and 8% of Mo–C>C wagging.336

When the purity of the ι(M–M) vibrations can be accepted as a valid approximation, force constants may be calculated for the M–M bonds by assigning the frequencies to a diatomic harmonic oscillator, M2. The values so obtained, some of which are listed in Table 16.11, are useful for comparative purposes even though they do not have absolute validity.

Table 16.11. ι(M–M) frequencies for multiply bonded dimetal species

Compound	ι(M–M) (cm-1) k (mdyne Å-1)a	ref.
	A. Quadruple bonds
(NEt4)4[Mo2(CN)8]	411	338
K4[Mo2Cl8]	345	331,333
K4[Mo2Cl8]·2H2O	345	331
Cs4[Mo2Cl8]	340	333
Rb4[Mo2Cl8]	338	333
(enH2)2[Mo2Cl8]·2H2O	348	331,333
(NH4)5[Mo2Cl8]Cl·H2O	350,338	331,333
[Mo2(CH3)8]4- in benzene	336	339
(C4H8ONH)2[Mo2Cl6(H2O)2]	357	340
(C4H8ONH)2[Mo2Br6(H2O)2]	350	340
(C5H5NH)2[Mo2I6(H2O)2]	340	340

778Multiple Bonds Between Metal Atoms Chapter 16

Compound	ι(M–M) (cm-1)	k (mdyne Å-1)a	ref.
Mo2(O2CH)4	406		341
Mo2(O2CH)4H2O	410		341
Mo2(O2CH)4(DMSO)2	360		341
Mo2(O2CCH3)4	404		324,342,
			343,344
Mo2(O2CCD3)4	403		344
Mo2(O2CCH3)4·2py	363		345
Mo2(O2CCF3)4	397		342,345
Mo2(O2CCF3)4·2py	367		342,345
[Mo2(O2CCH3)4]+ (gas)	360		268
CrMo(O2CCH3)4	393		346
Mo2[(CH2)2P(CH3)2]4	388		220
Mo2[O2C(2,4,6-Me3C6H2)]4	404		347
Mo2[O2C(4-CN-C6H4)]4	397		347
Mo2[O2C(4-MeO-C6H40]4	402		347
K2[Mo2(SO4)4]·2H2O	371		229,331,
			348,349
Mo2[PhNC(Ph)NPh]4	410		350
Mo2[(tol)NC(Ph)N(tol)]4	416		350
Mo2Cl4(PMe3)4	355	3.54	197,276,334,
			335,336
Mo2Br4(PMe3)4	352		197,276,335
Mo2I4(PMe3)4	343		197,276,335
Mo2Cl4(AsMe3)4	356		197,335
Mo2Cl4(PBun3)4	350	3.46	334,342
Mo2Cl4[P(OMe)3]4	347		342
Mo2Cl2(O2CPh)2(PBun3)2	392		347
Mo2Br2(O2CPh)2(PBun3)2	383		347
Mo2Br2[O2C(2,4,6-Me3C6H2)]2(PBun3)2	383		347
Mo2(OEP)2	341	3.29	351
Cr2(mhp)4	556	4.73	18
CrMo(mhp)4	504	5.03	313
Mo2(mhp)4	425	5.10	352
MoW(mhp)4	384	5.45	352
MoWCl4(PMe3)4	322(R),		334
	326(ir)
W2(O2CCH3)4	204		353
W2(O2CCMe3)4	313		353
W2(O2CCMe3)4(PPh3)2	287		353
W2(O2CCF3)4	310		354
W2(O2CCF3)4(PPh3)2	280		354
W2(mhp)4	284	4.71	18
W2Cl4(PBun3)4	260	3.65	334,355
Re2(O2CR)4Cl2b	288-295		330,342
Re2(O2CR)4Br2b	277-284		330,342
Re2(O2CCH3)2Cl4·2H2O	279		330,342
Re2(O2CCH3)2Br4·2H2O	277		330,342
(Bun4N)2[Re2F8]	318	5.55	204,356,357

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Compound	ι(M–M) (cm-1)	k (mdyne Å-1)a	ref.
(Bun4N)2[Re2Cl8]	272,275	4.12	204,280,342,
			357,358
(Bun4N)2[Re2Br8]	276	4.18	204,342,357, 358
(Bun4N)2[Re2I8]	257		204,357,359
Re2Cl6(PPrn3)2	278		342
Re2Cl6(PPh3)2	278		360
Re2Br6(PPh3)2	285		360
Re2Cl6[Me2N)2CS]2	276		342
[Mo2(HPO4)4]4-	345		287
Tc2(O2CCH3)2Cl4(CH3C(O)NMe2)2	290	3.38	237
Re2(O2CCH3)2Cl4(CH3C(O)NMe2)2	265		237
Mo2(O2CH)4L2	350-361		361
(L = various aromatic amines)
	B. Lower bond orders
K3Mo2(SO4)4·3.5H2O	373,385c		235,348
K4[Mo2(SO4)4]Cl·4H2O	369		349
K4[Mo2(SO4)4]Br·4H2O	370		349
[Mo2(HPO4)4]3-	352		287
Cs2[Mo2(HPO4)4]·2H2O	358		229,287
(C5H5NH)3[Mo2(HPO4)4]Cl	361		229
Re2Cl5(MeSCH2CH2SMe)2	267		342
Re2Cl5(PEtPh2)3	277		362
[Re2(OEP)2]+ (in THF)	290	4.61	351
Ru2(O2CH)4Cl	331,339		244,245
K[Ru2(O2CH)4Cl2]	335		244
Ru2(O2CCH3)4Cl	326		242,245
[Ru2(O2CCH3)4(H2O)2]+	326		245
Ru2(O2CEt)4Cl	338		242,245
Ru2(O2CPr)2Cl	328,331		242,245
Ru2(O2CCH3)4Br	321		242
(Bun4N)[Ru2(O2CEt)4Br2]	325		242
Ru2(O2CPr)4Br	329		242
Ru2(OEP)2	285	2.42	351
[Ru2(OEP)2]+ (in THF)	301	2.70	351
[Ru2(OEP)2]2+ (in THF)	310	2.86	351
(NBu4)2[Os2Cl8]	285		247
(NBu4)2[Os2Br8]	287		247
(NBu4)2[Os2I8]	270		247
Os2(O2CCH2Cl)4Cl2	236		363
Os2(O2CC2H5)4Cl2	233		363
Os2(O2CC3H7)4Cl2	228		362
			363
Os2(O2CCH3)4Cl2	229		364
Os2(O2CCD3)4Cl2	230		364
Os2(OEP)2	233	2.94	351
[Os2(OEP)2]+ (in THF)	254	3.46	351
[Os2(OEP)2]2+ (in THF)	266	3.79	351
[Rh2(O2CCH3)4Br2]−	286		365

780Multiple Bonds Between Metal Atoms Chapter 16

Compound	ι(M–M) (cm-1) k (mdyne Å-1)a	ref.
[Rh2(O2CCH3)4I4]−	314	365
Rh2(O2CCH3)4(PPh3)2	289	366,367,368,369
Rh2(O2CCH3)4(AsPh3)2	297	370,371
Rh2(O2CCH3)4(SbPh3)2	307	367,371
Rh2(O2CCH3)4(PPh3)4	226	372
Rh2(O2CH)4(PPh3)2	286	368
Rh2(O2CC2H5)4(PPh3)2	287	368
Rh2(O2CC3H7)4(PPh3)2	299	368
Rh2(CH3CONH)4(PPh3)2	275	369,371
Rh2(CH3CONH)4(AsPh3)2	283	369,371
Rh2(CH3CONH)4(SbPh3)2	294	371
Rh2(CF3CONH)4(PPh3)2	277	369
[Rh2(O2CCH3)4(PPh3)2]+	302	369
[Rh2(CH3CONH)4(PPh3)2]+	264	369
[Rh2(CH3CONH)4(AsPh3)2]+	283	369
[Rh2(CF3CONH)4(PPh3)2]+	277	369
Rh2(O2CCH3)4	355d	373
Rh2(O2CCH3)4(H2O)2	~340d	371

aForce constants in md Å-1 are calculated from k = (5.889 H 10-7)ι2µ, where 10 pt is the frequency in cm-1 and µ = MAMB /(MA + MB) with MA and MB representing atomic masses in Daltons.

b R may be CH3, C2H5, C3H7, C6H11 or C6H5.

c There are two crystallographically distinct [Mo2(SO4)4]3- units in the solid.

dThese values are far higher than any other ι(Rh–Rh) reported, but they occur in compounds with the shortest Rh–Rh bonds, viz. 2.385 Å in Rh2(O2CCH3)4(H2O)2 compared to 2.450 Å in Rh2(O2CCH3)4(PPh3)2.

The data in Table 16.11 provide some useful comparisons. For example, within the series of five MM'(mhp)4 compounds, with M and M' representing Cr, Mo or W, as well as MM'Cl4(PR3)4 (M,M' = Mo, W), we see that the mixed metal species, especially the MoW ones, have bonds that are stronger than would be predicted by linear interpolation between the homonuclear species.

A number of data in Table 16.11 show that axial ligands appreciably lower the stretching frequencies of M–M quadruple bonds. For example, for Mo2(O2CR)4 molecules, the axial ligands lower ι(Mo–Mo) by 30-40 cm-1 even though the Mo–Mo bond lengths change by only c. 0.02 Å.

When electronic transitions are examined at low temperatures with sufficient resolution, they often display vibrational fine structure, as we have already noted in Section 16.4. For an allowed transition, such as βΑβ*, the vibrational progressions should be in the totally symmetric skeletal modes, i.e. in ι(M–M), in the totally symmetric M–L stretching mode, ι(M–L), and in the totally symmetric M–M–L bending mode, β(M–M–L). The extent to which each of these contributes depends on how much the electronic excitation alters the internal coordinate (that is, d(MM), d(ML) or <MML) involved in the vibrational mode. For a βΑβ* transition the main effect is on d(MM) and hence strong progressions in ι(M–M) (or its combination with another mode) are the predominant vibrational feature. Thus, since much of the work on electronic spectra has dealt with βΑβ* transitions, a body of data has accumulated on the frequencies of M–M stretching in the state 1A2u, arising from the μ2/4ββ* configuration. Some of these are listed in Table 16.12 where they are designated ι'(M–M) to distinguish them from the ground state ι(M–M) values. A βΑβ* promotion lowers the M–M stretching frequency by amounts ranging from 10 to 50 cm-1.

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Table 16.12. Metal–metal stretching frequencies in the ground state and βΑβ* excited state,a ι(M–M) and ι'(M–M), respectively, in cm-1.

Compound	ι(M–M)	ι'(M–M)	ι–ι'	ref
[Mo2Cl8]4-	346	336	10	200,333
(NH4)4[Mo2Br8]	336	320	16	374
Mo2[(CH2)2P(CH3)2]4	388	345	43	220
K3[Mo2(SO4)4]·3.5H2O	373,385	350,357	25	222,223,348
K3[Mo2(HPO4)4]	352	334	18	287
(C4H8ONH)2[Mo2Cl6(H2O)2]	357	320	27	340
(C4H8ONH)2[Mo2Br6(H2O)2]	350	320	30	340
(C5H5NH)2[Mo2I6(H2O)2]	340	320	20	340
Mo2(O2CH)4	403	360	43	122,375
Mo2(O2CCH3)4	406	370	36	212,331,342
Mo2(O2CCH3)4 (in argon matrix)	–	390	–	215
Mo2(O2CCF3)4	397	356	41	212,342,345
Mo2(O2CCF3)4py2	367	331	36	376
Mo2Cl4(PMe3)4	358	335	22	276
Mo2Br4(PMe3)4	353	340	13	276
Mo2I4(PMe3)4	345	320	25	276
cis-[Mo2(mhp)2Cl2(PEt3)2]	–	370	–	225
Ru2(O2CH)4Cl	331	280	51	244
K[Ru2(O2CH)4Cl2]	335	312	23	244
(NBun4)2[Re2Cl8]	272	247	25	104,358
(NBun4)2[Re2Br8]	275	255	20	201,280,
				358,203
(NBun4)2[Re2I8]	257	240	17	377
K3[Tc2Cl8]·2H2O	370	320	50	199,221
Tc2(hp)4Cl	383	337	46	221
(NBun4)2[Os2Cl8]b	285	195	90	247
(NBun4)2[Os2Br8]b	287	211	76	247
(NBun4)2[Os2I8]b	270	183	87	247

a Unless otherwise stated.

b In these the excitation is believed to be βΑ/*.

Most values of ι' have been obtained in low-temperature studies of crystalline compounds. Under these conditions it seems likely that the 1A2u excited state is constrained to remain in an eclipsed or nearly eclipsed conformation. In the case of (NBun4)2[Re2Cl8] there is direct evidence for this.280 For [Re2Cl8]2- in solution, a time-resolved RR study272 has allowed measurement of the ι(Re–Re) vibration for the relaxed, staggered excited state, and this frequency, 262 cm-1 is intermediate between those for the eclipsed ground state (276 cm-1) and the eclipsed 1A2u (ββ*) state (249 cm-1). This seems reasonable and by an empirical rule relating bond lengths to force constants it has been estimated that the Re–Re distances are 2.239, 2.276, and 2.320 Å for the 1A1g(β2), staggered excited, and 1A2u (ββ*) states, respectively.

16.6.2 M–L stretching vibrations

Of the many other modes of vibration for M2X8, M2(O2CR)4L2 and similar species, those of next most interest, after ι(M–M) and ι'(M–M) are the metal-ligand stretching modes for the equatorial ligands, especially the totally symmetric ones.

782Multiple Bonds Between Metal Atoms Chapter 16

In the [Re2X8]2- ions and Mo2X4L4 molecules the totally symmetric M–X frequencies have been observed by Raman spectroscopy. The results are given in Table 16.13. These values are approximately those one would expect from those known in classical MX6n- and MX4n- complexes.

Table 16.13. Some metal–ligand stretching frequenciesa

	Compound	M–L	ι (cm-1)	ref.
	[Mo2Cl6(H2O)2]2-	Mo–Cl	325	265
	[Mo2Br6(H2O)2]2-	Mo–Br(?)	168	265
	[Mo2I6(H2O)2]2-	Mo–I	149	265
	Mo2Cl4(PMe3)4	Mo–Cl	274 (284)b	197,276b
		Mo–P	235	197
	Mo2Cl4(AsMe3)4	Mo–Cl	278	197
		Mo–As	217	197
	Mo2Br4(PMe3)4	Mo–Br	159 (165)b	197,276b
		Mo–P	235	197
	Mo2I4(PMe3)4	Mo–I	105c (143)b	197c,276b
		Mo–P	217	197
	[Re2F8]2-	Re–F	623.5d	204,357
	[Re2Cl8]2-	Re–Cl	361	204,245
	[Re2Br8]2-	Re–Br	211	204,245
	[Re2I8]2-	Re–I	152	204
a	These are the totally symmetric modes in the ground state unless otherwise noted.
b From emission spectra.
c	Doubtful value. Also band at 148 cm-1 that agrees better with emission value.
d IR-active modes at 568, 560, 552 cm-1.

The assignment of M–O stretching frequencies, especially the totally symmetric one, in M2(O2CR)4 molecules has been of importance in connection with assigning electronic absorption bands. In several cases, e.g., the Rh2(O2CR)4L2 molecules (see Section 16.4.3), transitions to /*(M–O) orbitals are prominent and the assignment of such transitions can be supported by the observation of progressions in the totally symmetric M–O stretching mode of the excited state.

It is not always obvious which frequency should be assigned to ι(M–M) and which to the totally symmetric ι(M–O) mode since the two often occur in the same frequency range. Some representative data are collected in Table 16.14.

Table 16.14. Some totally symmetric M–O stretching frequencies in M2(O2CR)4L2 molecules

Compound	Frequency (cm-1)	ref.
Mo2(O2CH)4L2a	350-400	341
Mo2(O2CCH3)4	323	344,328
[Ru2(O2CCH3)4(H2O)2]+	371	245
Ru2(O2CH)4Cl	432,440	243,245
Ru2(O2CCH3)4Cl	371	245
Ru2(O2CC2H5)4Cl	395	245
Ru2(O2CC3H7)4Cl	377,435	242,245

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Compound	Frequency (cm-1)	ref.
Os2(O2CCH3)4Cl2	393	364
Os2(O2CCD3)4Cl2	375	364
Os2(O2CCH2Cl)4Cl2	271	363
Os2(O2CC2H5)4Cl2	321	363
Os2(O2CC3H7)4Cl2	256	363
Rh2(O2CCH3)4(CH3CN)2	344	365
Rh2(O2CCH3)4(H2O)2	342	365
[Rh2(O2CCH3)4Cl2]2-	342	365
[Rh2(O2CCH3)4Br2]2-	338	365
[Rh2(O2CCH3)4I2]2-	338	365
Rh2(O2CH)4(PPh3)2	402	368
Rh2(O2CCH3)4(PPh3)2	338	368
Rh2(O2CC2H5)4(PPh3)2	310	368
Rh2(O2CC3H7)4(PPh3)2	289	368
Rh2(18O2CCH3)4(PPh3)2	332	367
Rh2(O2CCD3)4(PPh3)2	325	367

a See text.

It is evident that the frequency of the totally symmetric M–O stretching mode (and, presumably, the other ι(M–O) modes) is enormously sensitive to the mass of the group R in M2(O2CR)4L2 compounds. Thus in the Rh compounds, Rh2(O2CR)4(PPh3)2 through the series R = H, CH3, C2H5, C3H7, the frequency drops: 402, 338, 310, 289 cm-1, respectively. Simply replacing CH3 by CD3 changes the frequency from 338 to 325 cm-1. Similar changes occur with the other metals.

16.7 Other types of Spectra

16.7.1 Electron Paramagnetic Resonance

EPR spectra have not played a general role in the characterization of compounds with metalmetal bonds because relatively few of them are suitable for EPR study. However, in certain cases, EPR spectra afford valuable information. Unless otherwise noted, spectra mentioned were recorded at X-band frequency (c. 9.5 GHz)

Seven-electron Compounds.

The Mo2Cl83- ion is short-lived and its EPR spectrum has not been detected.378 However, numerous paddlewheel Mo25+ species have been observed. The first report378 of an Mo2(O2CR)4+ ion was for the electrogenerated ion with R = n-C3H7. The spectrum indicated that the unpaired electron is delocalized over both Mo atoms and was fitted with g = g = 1.941. An X-band spectrum379 of the ion with R = 2, 4, 6-triisopropylphenyl, showed a resonance at g = g = 1.936 with coupling to both Mo atoms, while a W-band (92.5 GHz) spectrum,380 at 10 K showed gxx = 1.9310, gyy = 1.9358 and gzz = 1.9427, although resolution of gxx and gyy was uncertain and would not be expected for axial symmetry.

A few other Mo25+ species have given EPR spectra. The [Mo2(SO4)4]3- ion was one of the

earliest,381 for which g = 1.891 and g	= 1.901. These g values are consistent with the μ2/4β
configuration.382 The [Mo2(HPO4)4]3−	ion has a very similar spectrum with g = 1.886 and
g = 1.894.287

784Multiple Bonds Between Metal Atoms Chapter 16

Data for Cr25+ and V23+ have only recently become available. The paddlewheel complex {Cr2[PhN)2CN(CH2)4]4}PF6 has been examined at W-band frequency (95 GHz) at 295 K and found to have g = 1.9701 and g = 1.9767.380 The anion [V2(DPhF)4]- displays an X-band spectrum at 6 K which consists of 15 hyperfine components (coupling to two 51V nuclei each with I = 7/2) with gav = 1.9999,383 thus showing one unpaired electron delocalized over the V23+ core.

Nine-electron compounds.

The [Tc2Cl8]3- ion provided the first example of the μ2/4β2β* configuration, and the most conclusive evidence for its authenticity is undoubtedly its EPR spectrum.384 Liquid solutions gave no spectrum, but certain frozen solutions (c. 10-3 M in a mixture of aqueous HCl and ethanol) at 77 K gave good spectra. Both X- and Q-band spectra were obtained. These spectra showed unequivocally the presence of one unpaired electron with hyperfine coupling to two equivalent 99Tc ( I = 9/2) nuclei. Analysis afforded the following parameters: g|| = 1.912,

g = 2.096, |A||| = 166 x 10-4 cm-1, and |A | = 67 x 10-4 cm-1. The qualitative facts, g|| < 2.00 and g > 2.00, have been shown382 to be consistent with the assignment of the unpaired elec-

tron to the β*-orbital, although it cannot be said that they uniquely demand this assignment. Numerous Re25+ compounds have been shown to have EPR spectra consistent with the μ2/4β2β* configuration. These, which have all been discussed in Chapter 8, include Re2Cl83-, [Re2(O2CR)4X2]-, [Re2X4(PR3)4]+, [`-Re2Cl4(diphos)2]+, `-Re2Cl5(dppm)2, inter alia. Because of the large number of hyperfine components, detailed interpretation of the signals in elusive, but

they are always consistent with delocalization of one electron over the two rhenium atoms.

It is uncertain whether the configuration in [Os2(hpp)4Cl2]+ is μ2/4β2β* or μ2/4β2/*, but the observed EPR signal385 is very unusual. It appears at a g value of 0.8 which is consistent with the bulk magnetic susceptibility, but the line width is about 6000 G.

Eleven-electron compounds.

The EPR spectra of Ru25+ and Os25+ compounds 386-389 are all affected by very large zerofield splitting of their S = 3/2 ground states, which has made complete development of the spin Hamiltonian impossible. The g values for the Ru25+ compounds are generally 2.1 to 2.2 for S = ½. Similar results were obtained for a few Os25+ compounds.390,391

Thirteen-electron compounds.

These are the compounds of cobalt, rhodium and iridium with M25+ cores. For M24+ cores, there is a metal–metal bond of order one, based, unambiguously, on a μ2/4β2β*2/*4 configuration. In many cases, stable singly oxidized species, where the configuration is probably μ2/4β2β*2/*3 (but might be μ2/4β2/*4β*) have been studied by EPR spectroscopy. The EPR spectrum of the electrochemically generated (but not isolated) [Co2(PhNCPhNPh)4]+ ion shows a signal at g|| = 1.98, split into 15 equally spaced lines by two cobalt atoms, each with I = 7/2.392 The compound Ir2(DAniF)4(O2CCF3), which was isolated and structurally characterized,393 has

an EPR spectrum in frozen CH2Cl2 (-100 °C) with giso of 2.14.

Compounds containing the Rh25+ core are very numerous and have been extensively studied. These have been cited in Chapter 12 where literature references that need not be repeated here were given. Many of these compounds show axial spectra, which have 2.05 ) g ) 2.09 and 1.91 ) g|| ) 1.98. The g|| component nearly always shows hyperfine coupling to one or both Rh(Is = 1/2) nuclei, depending on whether the Rh25+ core is in a symmetric environment that allows the unpaired electron to be delocalized or whether the electron is constrained to only one rhodium atom. For example the symmetric [Rh2(PhNCPhNPh)4]+ ion displays a triplet

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(A|| = 19.5x10-4 cm-1)394 while in the unsymmetrical (3,1) Rh2(ap)4Cl molecule there is a dou-

blet (A|| = 20.5 x 10-4 cm-1)395. Such spectra are believed to be due to a μ2/4β2/*4β* electron configuration.162,396,397

For many [Rh2(OCCH3)4L2]+ with L = H2O, CH3OH, THF, CH3CN and (CH3)2CO the g values, which are 0.6 ) g ) 1.87 and 3.38 ) g|| ) 4.00 have been interpreted as evidence for a μ2/4β2β*2/*3 configuration.396,397

Fifteen-electron compounds.

Some compounds of the elements Ni, Pd and Pt with M24+ cores are known and have M–M bond orders of zero. Some of the M25+ species have been obtainable by oxidation,398 and their EPR spectra are consistent with a μ2/4β2β*2/*4μ* configuration. In the case of [Ni2(DTolF)4]+ in frozen CH2Cl2, the X-band spectrum clearly shows that it is a metal-centered radical with axial symmetry (g|| = 2.038 and g = 2.210). Under the same conditions the palladium analog displayed only a symmetric line with g = 2.014 and it was proposed on this basis, plus structural evidence, that the unpaired electron might be delocalized essentially on the ligands.398 However, later work on a powder sample at 10 K, with a 92.5 GHz field revealed g|| = 1.9945 and g = 2.0202, thus supporting a metal-centered radical here too.399 It has also been reported that the Pd2[(PhNCPhNPh)4]+ ion has an axially symmetrical EPR signal with g|| = 1.98 and

g= 2.17.400

Another fifteen-electron configuration is found in Rh2[(PhN)2CPh]4-, generated electro-

chemically.394 It has g = 2.181 and g|| = 2.003 (triplet) and indicates that the odd electron is in a symmetrical μRhRh orbital.

Irradiation of Rh2(O2CR)4 compounds with gamma rays gave unstable species with EPR spectra consistent with the presence of a lone μ* electron.401

Miscellaneous.

The trigonal molecule Fe2(DPhF)3 has a ground state with seven unpaired electrons (µeff = 7.81 BM). Consistent with this, it has an EPR spectrum in frozen toluene glass that presents two signals corresponding to g values of 1.99 and 7.94.402

16.7.2 X-Ray spectra, EXAFS, and XPS

Core electron binding energies of a variety of dinuclear multiply-bonded complexes, which have been recorded using the X-ray photoelectron spectroscopic technique (XPS), are consistent with those expected for low-valent “electron-rich” metal centers. Extensive Re 4f and Mo 3d binding energy data are available for quadruply-bonded dirhenium403-407 and dimolybdenum408,409 complexes. It was suggested410 on the basis of Mo 3d XPS studies that Mo-silica catalysts prepared from Mo2(O2CR)4 may have different metal-support interactions from other Mo catalysts. In the case of chlorine-containing dinuclear complexes, measurements of the Cl 2p binding energies can be used to distinguish between terminal and bridging M–Cl bonds, since the core electron binding energies of these two environments fall in the order Clb > Clt.411,412

X-Ray emission spectra can, in principle, provide detailed information on the valence shell configuration, but such spectra suffer from poor resolution and only one such study has been reported. The Mo L`2, 15 X-ray emission spectrum of K4Mo2Cl8 is consistent with the order μ, /, β for the Mo–Mo bond.413

EXAFS measurements 414 on the Mo24+ ion in aqueous CF3SO3H gave an Mo–Mo distance of 2.12 Å and Mo–O distances of 2.14 Å, both in reasonable accord with expectation. An EXAFS study415 of the [Ru2(OEP)2]n species with n = 0, +1, + 2 has given Ru–Ru distances of 2.40 Å (2.408 Å by crystallography), 2.29 Å and 2.24 Å, respectively. The L2 and L3 absorption edge

786Multiple Bonds Between Metal Atoms Chapter 16

spectra were also measured for these species and interpreted to indicate a separation of c. 2 eV between the μ* and /* energies. The EXAFS and Re L3 absorption edge have also been measured357 for [Re2F8]2-, the former giving Re–F and Re–Re distances in reasonable agreement with those subsequently determined by crystallography.

The XPS spectra of Mo2Cl4(PMe3)4, MoWCl4(PMe3)4, and W2Cl4(PMe3)4 have been measured334 and interpreted to indicate that in the heteronuclear compound there is a net shift of electron density from W to Mo. Several other XPS studies of Cr24+ and Mo24+ compounds have

been reported.375,416,417,418

Nonlinear optical properties have been reported for M2Pd2Cr2(pyphos)4 molecules (M = Cr, Mo)419 and for Mo2 and W2 compounds of the M2(O2CBut)4, M2Cl4(PMe3)4, M2(OR)6 and M2(NMe2)6 types.420

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