Multiple Bonds Between Metal Atoms / 04-Molybdenum Compounds

Table 4.5. Compounds with anionic N,N bridging ligands

											Compound	Crystal	Virtual		r(Mo–Mo)	Twist	ref.
												sym.	sym.		(Å)	Angle (°)
Mo2[EtC(O)Npy]4												1	D2d		2.083(1)	50	249
												1	D2d		2.087(1)	50
												1	D2d		2.081(1)	50
Mo2(map)4·2THF												1	D2d		2.070(1)	1.6	239
Mo2(PhNpy)4												¯	C2h		2.073(2)	0	240
												1
												¯	C2h		2.068(2)	0
												1
Mo2[(PhN)2CPh]4												2	D4h		2.090(1)	50	250
Mo2{[(p-tol)N]2CH}4												4	D4h		2.085(4)	3.2	251
Mo	2	(N		Ph			2	)	·1/	C H	8	1	D	4h	2.083(2)	10.5	252
			3					4	2	7		¯
Mo2(DPhF)4													D4h		2.094(1)	zero	253
												1
Mo2(D			3,5-Cl2					PhF)4				¯	D4h		2.096(1)	zero	253
												1
Mo2(D			m-Cl			PhF)4						¯	D4h		2.097(1)	zero	254
												1
Mo2(DAniF)4												¯	D4h		2.096(1)	zero	254
												1
Mo2(D			p-Cl		PhF)4							¯	D4h		2.090(1)	zero	255
												1
Mo2(D			p-Br		PhF)4							¯	D4h		2.087(2)	zero	256
												1
Mo2(azin)4(Me2CO)2												¯	C2h		2.135(1)	zero	257
												1
Mo2(azin)4(THF)2												¯	C2h		2.124(1)	zero	257
												1
Mo2(ambt)4·THF												¯	C2h		2.103(1)	0	135
												1
Mo2(acbt)4·THF												¯	C2h		2.117(1)	0	135
												1
[(C7H7)NH3][Mo2(µ-(HNC(CH3)NC7H7))(CH3CN)6](BF4)4·3CH3CN												1	Cs		2.157(1)	NR	242
trans-Mo2(O2CCH3)2[PhC(NSi(CH3)3)2]2												2	D2h		2.069(1)	NR	144
cis-Mo2(O2CCH3)2[PhC(NSi(CH3)3)2]2												2	C2v		2.124(1)	NR	144
Mo2(DPhIP)4												1	D2h		2.114(1)	NR	258
Mo2(DPhIP)4(CH3CN)(CuCl2)2·2CH3CN												1	D2h		2.078(1)	NR	258
Mo2(DPhIP)2(O2CCH3)2												¯	C2h		2.089(1)	NR	258
												1
Mo2(DpyF)4												¯	D4h		2.110(1)	0	259
												1
[Mo2(DpyF)4Co][CoCl4]												2	C2v		2.115(5)	NR	259

Cotton		Compounds Molybdenum
	99

	Compound	Crystal	Virtual	r(Mo–Mo)	Twist	ref.
		sym.	sym.	(Å)	Angle (°)
	[Mo2(DpyF)4Cu4Cl2](CuCl2)2	2	D2d	2.127(1)	NR	259
	Mo2(HBPAP)4	1	D2d	2.081(1)	NR	260
	[Mo2(O2CCH3)2(pynp)2](BF4)2	1	C2	2.124(1)	50	149
	cis-[Mo2(mphamnp)2(O2CCH3)2]·C5H12	2	C2	2.097(2)	NR	150
	Hmphamnp = 2-acetamido-5-methyl-7-phenyl-1,8-naphthyridine
	trans-[Mo2(mbznnp)4]	1	D2d	2.091(3)	c. 1	150
	mbznnp = 2-benzylamino-7-methyl-1,8-naphthyridine
	cis-[Mo2(mphonp)4]·Et2O	¯	C2h	2.079(2)	zero	150
		1
	Hmphonp = 5-methyl-7-phenyl-1,8-naphthyridin-2-one
	trans-[Mo2(mphonp)4]·Et2O	1	D2d	2.084(1)	c. 3	150
	Mo2(µ-dpa)4	2	D2d	2.097(1)	3.4	64
	Hdpa = bis(2-pyridyl)amine
	Mo2(TPG)4	2	D4h	2.084(1)	4.5	261
	Mo2(hpp)4	¯	D4h	2.067(1)	zero	262
		1
	cis-Mo2(DAniF)2(calix)	1	C2v	2.118(3)	50	263
				2.122(3)	50
				2.125(3)	50
				2.127(3)	50
	trans-{Mo2[(C6H5N)2CH]2py4}(BF4)2	¯	D2h	2.107(2)	50	264
		1
	{Mo2[(C6H5N)2CH](CH3CN)6}(BF4)3	1	C2v	2.149(1)	50	264
		1	C2v	2.151(1)	50
	cis-{Mo2(C6H5N)2CH]2(CH3CN)4}(BF4)2	1	C2v	2.146(1)	50	264
	cis-{Mo2[(p-MeOC6H4N)2CH]2(CH3CN)4}(BF4)2	1	C2v	2.144(1)	50	264
	{Mo2[(p-MeOC6H4N)2CH](CH3CN)6}(BF4)3	1	C2v	2.152(1)	50	265

	100
4 Chapter		Bonds Multiple
		Atoms Metal Between

Molybdenum Compounds 101

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The ligands of type 4.16 come in a variety of shapes and sizes, but the largest class is the amidinates, in which X is C–H or C–R. The former are called formamidinates and there are more of these than any other kind. All amidinate complexes of Mo24+ are fairly easily made by reaction of Mo2(O2CCH3)4 with the amidinate anion although other preparative reactions are known. The amidines themselves (or their anions) are also easy to make from carbodiimides according to the reaction:

R'

R'Li + RNCNR Li NRCNR

For the special case of diaryl formamidinates, 4.17, the use of triethylorthoformate and a primary amine allows for a very wide choice of substituents on the nitrogen atoms, as shown in the following reaction:

	HC(OEt)3 + 2	NH2	-3EtOH		H	X
				X	C
	X			N		N
				H
					4.17

While practically all the amidinates that have been used are symmetrical, unsymmetrical ones, PhNC(H)N(2-py) being an important example, can be made in other ways.266

Other ligands mentioned in Table 4.5 are triazinates, RN3R−, 2-aminopyridines, especially anilinopyridine (pyNPh), 7-azaindole, 4.18 (azin), and map, the amino analog of mhp. These compounds have no special features, although the azin ligand gives relatively long Mo–Mo bonds. In addition to Mo2(Ph2N3)4 which is of known structure, the Mo2(tol2N3)4 compound is also known.97 It was made in an unusual way, as shown in the following reaction:

Mo2R2(NMe2)4 + 4(p-tol)N(H)NN(p-tol) Α Mo2[N3(p-tol)2]4 + 4HNMe2 + alkane + alkene

N N

4.18

The first reported Mo2(amidinate)4 compound contained the N,N'-diphenylbenzamidinate ligand, (PhN)2CPh−. This and its di-p-tolyl analog were obtained by reacting the amidine with Mo(CO)6 in a refluxing hydrocarbon.250 Both products displayed strong resonance-enhanced Raman lines at 412 cm−1, indicative of the quadruple bonds present. Subsequent work251,267 showed that this synthetic method was generally valid, especially for formamidines. However, an alternative method in which a formamidinate anion reacts with Mo2(O2CCH3)4 is now generally preferred.

The most thoroughly investigated Mo2(amidinate)4 compounds are those in which the amidinate is a diarylformamidinate,268 of the type 4.17. As the six entries in Table 4.5 show, the Mo–Mo distance is essentially insensitive to the substituents on the aryl groups, even though the Hammett μ parameters cover an enormous range, from −0.27 to +0.74. It is also true that the HOMO–LUMO (β–β*) gap is essentially insensitive to the changes in aryl groups. How-

102Multiple Bonds Between Metal Atoms Chapter 4

ever, the absolute energy of the HOMO is very sensitive and this shows up dramatically in the oxidation potentials measured electrochemically, as will be discussed fully in Section 4.4.2.

The anionic ligand DPhIP, 4.19, forms a paddlewheel complex,258 Mo2(DPhIP)4, in which the Mo–Mo distance is long compared to those in other Mo24+ paddlewheel complexes with N,N bridging ligands. As shown in Fig. 4.13(a), the longer-than-expected Mo–Mo distance (i.e., 2.114(1) Å instead of about 2.07 Å) may be attributed to donation of lone-pair electron density of the four non-bonded nitrogen atoms into the /* orbitals. When two CuI atoms are introduced, as shown in Fig. 4.13(b), they become the receptors for this electron density and the Mo–Mo distance decreases to 2.078 Å.

	Ph	N	Ph
	N		N
		4.19

Fig. 4.13. (a) The Mo2(DPhIP)4 molecule. (b) The [Mo2(DphIP)4Cu2(CH3CN)]2+ cation. The four N Α Mo dative interactions in (a) are replaced by N Α Cu bonds in (b) thereby decreasing the Mo–Mo distance from 2.114(1) Å to 2.078(1) Å.

With the bridging ligand DpyF, 4.20, which has the potential to form several regioisomers of Mo2(DpyF)4, only one, in which all four ligands employ the two central nitrogen atoms, was isolated.259 The eight dangling py groups do not interact with the axial positions of the Mo24+ units. This molecule can, however, interact with additional cations (Co2+, Cu+) through its pyridyl nitrogen atoms to give the two compounds listed below it in Table 4.5. These acquired metal ions show little or no interaction with the central Mo24+ unit.

The Mo2(HBPAP)4 compound260 (see 4.21 for H2BPAP) as well as its chromium analog have paddlewheel structures in which four N–H hydrogen atoms are located close to the axial positions of the dimetal units. As a result of the large diamagnetic anisotropy of the M2 unit, their chemical shifts are c. 3 ppm upfield from where they would normally be expected.

Molybdenum Compounds 103

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		H		Ph	N	Ph
N		C	N	N		N
	N	N		H		H
		DpyF			H2BPAP
		4.20			4.21

Several Mo2L4 paddlewheels have been made in which L is a substituted naphthyridine.150 In only one case,149 [Mo2(O2CCH3)2(pynp)2]2+, where pynp represents 2-(2-pyridyl)-1,8-naphthy- ridine, does the napthyridine moiety itself bridge the metal atoms. Instead, in other cases one such nitrogen atom and an adjacent NR− or O− form an NCN or NCO bridging group.

Finally, there are two paddlewheel compounds in which the bridging groups are guanidinate anions, Mo2(TPG)4 and Mo2(hpp)4. The chief interest of both of these, particularly the latter, is the degree to which guanidinates stabilize the higher oxidation states, Mo25+ and Mo26+. This topic will be discussed at length in Section 4.4.2

4.2.3 Compounds with miscellaneous other anionic bridging ligands

Monoand dithiocarboxylates.

Many Mo2(OSCR)4 and Mo2(S2CR)4 compounds are known; among the former are those with R = CH3, Ph, C5H4FeC5H5269-271 and among the dithiocarboxylates are those with R = CH3, Ph and p-tolyl.269,272,273 In addition there are dithiocarbonates (xanthates) ROCS2− (R = Me, Et, Pri, Prn, Bun or CH2Ph),269,273-275 thioxanthates RSCS2− (R = Et, Pri, But or CH2Ph),273 and dithiocarbamates R2NCS2− (R = Et, Pri or Ph.)269,273 In most instances, these complexes are pre- pared269,273-275 by the direct reaction of Mo2(O2CCH3)4 with an alkali metal or ammonium salt for the appropriate ligand in methanol, ethanol or THF. Some syntheses, particularly for the Mo2(OSCR)4 and Mo2(S2CR)4 compounds, have been achieved269,270,273 through use of the free acids RCOSH and RCS2H. However, in the case of the odoriferous phenyland methyldithiocarboxylic acids it is preferable to react Mo2(O2CCH3)4 directly with the reagents CH3CS2MgBr and PhCS2MgBr without first converting the latter to the free acids or some suitable salt.272 The complexes Mo2(S2CPh)4 and Mo2(S2CC5H4FeC5H5)4 have been reported to form upon the slow thermal decarbonylation of the mononuclear species Mo(CO)3(S2CR)2.271

Crystal structure determinations on the tetrahydrofuran solvates Mo2(S2CR)4·2THF (R = CH3 or Ph) have confirmed272 that these are indeed quadruply bonded dimolybdenum(II) complexes with Mo–Mo distances of 2.133 Å and 2.139 Å, respectively. A lengthening of c. 0.04 Å compared to Mo2(O2CR)4 compounds may be attributed partly to the presence of two weakly bound axial THF molecules but must also reflect the steric and electronic properties of the RCS2− ligands. The similarity of the electronic absorption spectra270 of Mo2(S2CPh)4 and Mo2(OSCPh)4, together with mass spectral evidence for a dinuclear structure in the case of the monothiocarboxylates269,270 implies that a close structural relationship exists between Mo2(S2CR)4 and Mo2(OSCR)4. The structure of Mo2(OSCPh)4(OPPh3)2 shows the effect of axial coordination, with an Mo–Mo bond length of 2.152 Å.89

The xanthates, Mo2(S2COR)4, which, like the monothioand dithiocarboxylate derivatives are red in color, also exhibit the expected paddlewheel structure. A crystal structure determination on Mo2(S2COEt)4·2THF, has shown134 the presence of an eclipsed Mo2S8 skeleton and Mo–Mo distance (2.125(1) Å) which is only a little shorter than the Mo–Mo distance in Mo2(S2CR)4. While a definitive structure determination is not yet in hand for a thioxanthate

104Multiple Bonds Between Metal Atoms Chapter 4

derivative, the available spectroscopic characterizations273 on Mo2(S2CSR)4 are in accord with the expected ligand-bridged structure.

The xanthantes exhibit an interesting reaction chemistry which in some ways resembles that of Mo2(O2CCF3)4. The ethyl and isopropyl derivatives form 1:2 adducts with ligands such as pyridine, several of which are quite stable in the solid state,269,275 and Mo2(S2COEt)4 reacts with halide ions to form salts such as [Ph3PCH2Ph]2[Mo2(S2COEt)4X2] (X = Br or I) and {[Ph3PCH2Ph][Mo2(S2COEt)4Cl]}n.276

In the original synthesis of Mo2(S2COEt)4 by reacting Mo2(O2CCH3)4 with an excess of potassium xanthate, a green product of unknown stoichiometry was also isolated.274 Some time later this was shown276 to be a salt of the [Mo2(S2COEt)5]− anion. This species can also be prepared by reacting Mo2(S2COEt)4 with a stoichiometric amount of KS2COEt and precipitated as its [Ph4As]+ or [Ph3PCH2Ph]+ salt;276 the related isopropyl derivative [Mo2(S2COPri)5]− has also been prepared by this means.275 The mixed xanthates [Mo2(S2COR)4(S2COR')]− (R = Me, R' = Et; R = Et, R' = Me) together with [Mo2(S2COR)4(S2CR)]−and [Mo2(S2COR)4(OSCR)]− have also been obtained. Both dinuclear and tetranuclear276 structures have been proposed for the [Mo2(S2COR)5]− anions on the basis of their spectrosopic275,276 and conductance276 properties, but the structural questions have not yet been resolved by a crystal structure determination.

The reactions between Mo2(O2CCH3)4 and dialkyldithiocarbamates are more complicated than those involving the other sulfur ligands.269,273,277 While genuine quadruply-bonded Mo2(S2CNR2)4 compounds may indeed exist,269,273 and there is spectroscopic evidence273 in support of this contention, the most stable complexes isolated from this system are the green dimolybdenum(IV) complexes Mo2S2(S2CNR2)2(SCNR2)2, where R is Et or Pr. These novel complexes contain a bridging Mo2S2 sulfide unit, two conventional chelating dithiocarbamate ligands, and two thiocarboxamide ligands (SCNR2), the latter arising from cleavage of a C–S bond of each of two dithiocarbamates (see 4.22).277 The short Mo–C distance (2.069 Å) indicates277 carbenoid character in the Mo–C bond involving each of the thiocarboxamido functions, and a Mo–Mo distance of 2.705 Å implies a Mo–Mo interaction.

4.22

The complexes Mo2S2(S2CNR2)2(SCNR2)2 may be viewed as being derived from Mo2(S2CNR2)4 via an internal irreversible redox reaction whereby the metal is oxidized (MoII to MoIV) and two of the ligands are reduced. This reaction points to the existence of a rich and interesting redox chemistry for many species containing the [Mo2S8] core. Bromine and iodine react with stoichiometric amounts of Mo2(S2COR)4 (R = Et or Pri) in chlorocarbon solvents or THF to produce crystalline solids of composition Mo2(S2COR)4X2.278 These turn out not to be products of a ‘simple’ oxidative addition of X2 to a Mo–Mo quadruple bond, whereby a triple bond would result, but instead involve a major change in the bonding mode of all four xanthate ligands.278 From the structure determination of the dimolybdenum(III) complex Mo2(S2COEt)4I2 (see Fig. 4.14), two xanthate ligands were found to be chelating while the remaining two coordinate in an extraordinary bridging manner.278 Each of the latter may be considered to be acting as a bidentate, three-electron donor to one metal atom while at the same time contributing

Molybdenum Compounds 105

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four electrons, as a tridentate donor, to the other metal atom. The observed Mo–Mo distance of 2.720(3) Å probably corresponds to a bond order of one. Other examples of the oxidation of such complexes include the conversion of Mo2(S2CNR2)4 to compounds that contain the Mo2O34+ core,269,279,280 but in these instances the products do not contain a Mo–Mo bond.

Fig. 4.14. The structure of Mo2(S2COEt)4I2.

The only monothiocarbamate paddlewheel molecule is Mo2(OSCNPri2)4, prepared from Mo2(O2CCH3)4 and Li(OSCNPri2) in ethanol281 It has an Mo–Mo distance of 2.112(1) Å.

Dichloromethane solutions of xanthate, thioxanthate and dithiocarboxylate complexes exhibit similar electrochemistry,273 including a common quasi-reversible one-electron reduction in the potential range −1.4 to −2.2 V (versus SCE). A second reduction at more cathodic potentials is irreversible, the electron transfer being followed by dissociation of a ligand which is itself electrochemically active. The xanthates and thioxanthates are irreversibly oxidized at approximately +0.8 and +0.9 V, respectively.273

The dithiocarbamates exhibit a reduction in the vicinity of −2.1 V and an oxidation in the range +0.1 to +0.4 V. Controlled potential electrolysis of a dichloromethane solution of Mo2(S2CNPri2)4 at potentials anodic of the oxidation wave leads to the formation of Mo2(S2C- NPri2)2(SCNPri2)2, as identified by its characteristic cyclic voltammogram.273

Other compounds that may contain Mo–Mo quadruple bonds, although structural data are lacking, are (Ph4As)4Mo4(C4S4)4,282 and a substance formed upon reaction of K4Mo2Cl8 with (NH4)2MoS4 in 1 M aqueous KCl which has been formulated as K4[Mo2(MoS4)4].

A few thiophosphorus compounds are known.200,269 These include Mo2(S2PPh2)4 and Mo2(S2PEt2)4. The latter exists in isomeric forms. Unsolvated Mo2(S2PEt2)4 has two bridging and two chelating ligands whereas Mo2(S2PEt2)4·THF has a paddlewheel structure with an axial THF. The Mo–Mo distances are 2.137(1) Å and 2.123(1) Å, respectively. Apparently Mo2(S2PMe2)4 behaves similarly,283 but no bond distances have been reported. It is surprising that there should be so little difference between the Mo–Mo distances in the two structures.

There are compounds containing F2PS2− ligands,269 viz., Mo2(S2PF2)4, Mo2(S2PF2)2(O2CCF3)2 and Mo2(S2PF2)2(O2CCH3)2. NMR spectroscopy indicates paddlewheel structures for all three with trans configurations in the two mixed ligand compounds. Mo2(OSPEt2)4·THF is isostructural with Mo2(S2PEt2)4·THF with an Mo–Mo distance of 2.128(2) Å.200

4.3Non-Paddlewheel Mo24+ Compounds

4.3.1 Mo2X84− and Mo2X6(H2O)22- compounds

As early as 1965284 it was shown that the following reversible interconversions occur:

Re2Cl82− + 4RCO2H = ClRe(O2CR)4ReCl + 4HCl + 2Cl−

106Multiple Bonds Between Metal Atoms Chapter 4

Thus, when the Mo2(O2CCH3)4 structure was reported,1 the idea of proceeding in an analogous way to make the new anion Mo2Cl84−, which would be a stereoelectronic analog of Re2Cl82−, was soon shown to be valid. The first reported compound3 of the Mo2Cl84− ion was K4Mo2Cl8·2H2O. The Mo–Mo distance of 2.139 Å and the rigorously eclipsed rotational conformation attested to the existence of a quadruple bond between the molybdenum atoms. The structure, exactly as originally reported, is shown in Fig. 4.2. It is interesting to note that it was in this structure that the tendency of M2X8n− and related species to display a type of disorder in which some of the quasicubic M2X8 units are oriented at 90° to the principal orientation (in this case about 7%) was first observed. For an extended discussion of this type of disorder, see Section 16.1.5. Table 4.6 lists all Mo2X84− and Mo2X6(H2O)22− compounds for which crystal structures are known.

Table 4.6. Structures of [Mo2X8]4− and [Mo2X6(H2O)2]2− compounds

								Compound										Crystal	Virtual	r(Mo–Mo)		Twist	ref.
																		sym.	sym.	(Å)		Angle (°)
K4Mo2Cl8·2H2O																		2/m	D4h	2.139(4)		0	3
(enH2)2Mo2Cl8·2H2O																		¯	D4h	2.134(1)		0	285
																		1
(NH4)5Mo2Cl9·H2O																		m	D4h	2.150(5)		0	286
(pipH2)2Mo2Cl8·4H2O																		¯	D4h	2.129(3)		0	287
																		1
[	H3N(CH2)3NH3									] [	Mo2Cl8					]	·4H2O	¯	D4h	2.125(2)	53%	0	288
										2								1
																				2.123(2)	47%	0
[	H3N(CH2)4NH3									] [	Mo2Cl8					]		¯	D4h	2.132(2)		0	288
										2								1
(NH4)4Mo2Br8																		4/mmm	D4h	2.135(2)		0	290
(NH4)4Mo2(NCS)8·4H2O																		¯	D4h	2.162(1)		0	291
																		1
(NH4)4Mo2(NCS)8·6H2O																		¯	D4h	2.174(1)		0	291
																		1
																		¯	D4h	2.177(1)		0
																		1
Li4Mo2(CH3)8·4THF																		¯	D4h	2.148(2)		0	292
																		1
[	Bu	n		]			[	Mo2(CN)8			]		·8CHCl3					¯	D4h	2.122(2)		0	155
		4N			4													1
								[	Mo2Cl6(H2O)2						]			¯	C2h	2.118(1)		0	293
(morphH)2																		1
								[	Mo2Br6(H2O)2						]			¯	C2h	2.114(2)		0	293
(morphH)2																		1
(pyH)3[Mo2Br6(H2O)2]Br																		2	C2	2.130(4)		50	294
(picH)2				[		Mo2Br6(H2O)2								]				¯	C2h	2.122(2)		0	295
																		1
(pyH)2			[		Mo2I6(H2O)2							]						¯	C2h	2.115(1)		0	296
																		1
(picH)2				[		Mo2I6(H2O)2							]					¯	C2h	2.116(1)		0	297
																		1

The preparation of K4Mo2Cl8·2H2O triggered extensive investigations of reactions between Mo2(O2CCH3)4 and hydrohalic acids under a wide variety of experimental conditions, as will now be described. First, it should be mentioned that the conversions of Mo2(O2CR)4 to [Mo2X8]4− by halide ions proceed through the intermediacy of mixed halide-carboxylate species such as [Mo2(O2CR)4X2]2−, [Mo2(O2CR)3X3]2−, and [Mo2(O2CR)2X4]2−, which have been considered previously in Section 4.1.3.

The reactions of Mo2(O2CCH3)4 with hydrohalic acids (HCl, HBr) to produce Mo2X84− ions must be conducted under carefully controlled conditions or oxidative cleavage will occur, leading to [MoOX4]− (X = Cl, Br) anions,298,299 or [MoCl5(H2O)]2−.300

The conditions used to convert Mo2(O2CCH3)4 to K4Mo2Cl8·H2O,3 namely reaction at c. 0 °C in constant-boiling hydrochloric acid, were soon adapted to the synthesis of other such salts. These included (enH2)2Mo2Cl8·2H2O,285 where enH2 = H3NCH2CH2NH3, and (NH4)5Mo2Cl9·H2O,286 which were structurally characterized and shown to contain the

Molybdenum Compounds 107

Cotton

eclipsed [Mo2Cl8]4− anion (Fig. 4.2). The anhydrous salt K4Mo2Cl8 is formed, instead of the dihydrate, when the concentrated hydrochloric acid is saturated with HCl gas.301 Similar procedures were used subsequently by others to prepare Rb5Mo2Cl9·H2O,290 Rb4Mo2Cl8,302 and Cs4Mo2Cl8,302 while the salt (pipH2)2Mo2Cl8·4H2O (pip = piperazine) was isolated287 by reacting (morphH)2Mo2Cl6(H2O)2 (see below) with (pipH2)Cl2 in hydrochloric acid. This complex has a structure like that of other salts of the [Mo2Cl8]4−.287 Adaption of this general synthetic method to the related bromide systems by Brencˇic and co-workers290,303 and others101 has permitted the isolation of (NH4)4Mo2Br8, Cs4Mo2Br8, and (NH4)5Mo2Br9·H2O. In the case of (NH4)4Mo2Br8, the synthesis is actually best approached290 via the sulfate complex (NH4)4Mo2(SO4)4·2H2O, the latter being prepared by the reaction of (NH4)5Mo2Cl9·H2O with (NH4)2SO4 in cold 1 M sulfuric acid.

(NH4)5Mo2Br9·H2O and Rb5Mo2Cl9·H2O have been shown290 to be isostructural with (NH4)5Mo2Cl9·H2O and therefore they contain the [Mo2X8]4− anions. A crystal structure determination of (NH4)4Mo2Br8 has revealed290 the expected eclipsed [Mo2Br8]4− anion of D4h symmetry and a Mo–Mo distance of 2.135(2) Å. An attempt to synthesize K4CrMoCl8 by the reaction of CrMo(O2CCH3)4 with a solution of KCl dissolved in concentrated hydrochloric acid saturated with HCl gas afforded only K4Mo2Cl8·2H2O.304

The spectroscopic properties of salts of the [Mo2X8]4− anions, (X = Cl or Br) have been of considerable interest and importance and are discussed in some detail in Chapter 16. Of additional note are the 95Mo NMR spectra that have been reported for K4Mo2Cl8 and Cs4Mo2Br8·2H2O.96 In addition to salts containing the [Mo2Cl8]4− and [Mo2Br8]4− anions, various halide ‘deficient’ species have been isolated and structurally characterized. The first one to be isolated, K3Mo2Cl7·2H2O, was obtained by adding alcohol to solutions that would otherwise have produced K4Mo2Cl8·2H2O if allowed to crystallize slowly.301 On the other hand, Rb3Mo2Cl7·2H2O separates from constant boiling hydrochloric acid solutions that contain Mo2(O2CCH3)4 and RbCl without the addition of alcohol.301 A bromide analog, Cs3Mo2Br7·2H2O, was later prepared by Brencˇic et al.303 and found to crystallize in the same space group and to have similar cell dimensions as Rb3Mo2Cl7·2H2O. A full crystal structure has yet to be carried out on any of these alkali metal salts. However, the pyridinium salt (pyH)3Mo2Br7·2H2O, which was prepared by reacting (NH4)5Mo2Cl9·H2O in hydrobromic acid with pyridinium bromide, has been shown294 to be the double salt (pyH)3[Mo2Br6(H2O)2]Br. The [Mo2Br6(H2O)2]2− anion in this salt possesses a Mo–Mo distance of 2.130(4) Å and a structure as represented in 4.23. Whether the alkali metal salts have such a structure is unknown. Actually, the [Mo2X6(H2O)2]2− anions (X = Cl, Br or I) are particularly well characterized species, structure determinations having been carried out on several pyridinium and 4-methylpyridinium salts of the type (pyH)2[Mo2X6(H2O)2] and (picH)2[Mo2X6(H2O)2], as well as the morpholinium derivatives (morphH)2[Mo2X6(H2O)2] (Table 4.6).293,295-297 However, in all these instances the anions are centrosymmetric (4.24) and therefore differ from the structure of (pyH)3[Mo2Br6(H2O)2]Br, although in each the rotational geometry is eclipsed. Compared to the corresponding [Mo2X8]4− anions, the Mo–Mo distances in the [Mo2X6(H2O)2]2− species are shorter by up to 50.02 Å, no doubt reflecting the decreased anion charge in the latter species. Their preparation is quite straightforward,293,295,297,305 and involves halide exchange reactions in hydrohalic acid media in the presence of the appropriate amine hydrohalide in the case of the pyridinium and 4- methylpyridinium salts. Thus, (picH)2[Mo2Br6(H2O)2] is obtained from (NH4)5Mo2Cl9·H2O295 while (pyH)2[Mo2I6(H2O)2] and (picH)2[Mo2I6(H2O)2] are prepared via (pyH)3Mo2Br7·2H2O and (picH)2[Mo2Br6(H2O)2]2, respectively.296,297 The morpholinium salts are obtained from (morphH)4Mo2Cl8 and (NH4)4Mo2Br8.293 The electronic absorption, infrared, and Raman spectra of the series of complexes (morphH)2[Mo2X6(H2O)2] (X = Cl or Br) and (pyH)2[Mo2I6(H2O)2]

108Multiple Bonds Between Metal Atoms Chapter 4

have been studied in considerable detail;306 these properties accord with the presence of Mo–Mo quadruple bonds.

Br

OH2

2

X

X

2

Br

Br

OH2

X

Mo

Mo

Mo

Mo

H2O

Br

X

H2O

Br

Br

X

X

4.23

4.24

Although kinetic studies on the reaction of Mo2(O2CCH3)4 with halide ion have not been reported, the reverse reaction, namely, the reaction of acetic acid with equilibrated solutions of K4Mo2Cl8 in hydrochloric acid, p-toluenesulfonic acid, and mixtures of these two acids has been studied.41,307 Several mechanisms have been advocated for these reactions.

It is a little surprising that no compounds containing Mo2F84− or Mo2I84− ions have been reported. On the other hand the Mo2(NCS)84− ion291 and Mo2(CN)84− ion308-310 are well characterized. The Mo2Cl84− ion (as well as Mo2Cl8H3−) are reducing agents and deoxygenating agents, known to convert sulfoxides to sulfides.311

4.3.2 [Mo2X8H]3− compounds

The red or violet colored salts of the [Mo2X8]4− and [Mo2X6(H2O)2]2− anions that are formed from Mo2(O2CCH3)4 are the obvious non-redox halide substitution products of the carboxylate. However, unlike the corresponding substitution chemistry of Re2(O2CR)4Cl2, that of Mo2(O2CR)4 can also be more complicated. At around the time of the synthesis and structure elucidation of K4Mo2Cl8·2H2O,3 it was reported312 that the reaction between Mo2(O2CCH3)4 and RbCl or CsCl in deoxygenated 12 N hydrochloric acid at temperatures higher than those used to produce [Mo2Cl8]4−, namely 60 °C or thereabouts, afforded high yields of green-yellow Rb3Mo2Cl8 or Cs3Mo2Cl8. These would appear to be Mo(+2.5) derivatives and a crystal structure determination on Rb3Mo2Cl8, with which the cesium salt was found to be isostructural, led to the proposal312 that the binuclear anions were best described as confacial bioctahedra (M2X9) with one-third of the bridging halogen atoms absent. A similar structural situation was believed to exist with the bromide salt Cs3Mo2Br8 which was later prepared313 by an analogous procedure. Sheldon and coworkers had also described314 a series of salts containing the [Mo2X8]3− anions (X = Cl or Br). Their attempts to identify the molybdenum oxidation state by using the ferric-permanganate titration method was puzzling since solutions of these complexes in 4 −12 M hydrochloric acid gave oxidation numbers of +3 rather than +2.5.

Some years later, Rb3Mo2Cl8 and Cs3Mo2Br8 were found to be diamagnetic, a result inconsistent with the non-integral oxidation number of +2.5. These complexes were reinvestigated315 and reformulated as the dimolybdenum(III) species [Mo2X8H]3− on the basis of deuterium and tritium labeling experiments and infrared spectroscopy (ι(Mo–H–Mo) at 5 1260 cm−1).315 Accordingly, the overall reaction of Mo2(O2CCH3)4 with the hydrohalic acids may be represented as follows:

Mo2(O2CCH3)4 + 8HX Α [Mo2X8H]3− + 3H+ + 4CH3CO2H

This reaction appears to be quantitative when carried out at temperatures of 60 °C and above, and with the exclusion of oxygen. It constituted the first example of an oxidative-addition reaction involving a well-defined metal–metal bond.