Multiple Bonds Between Metal Atoms / 04-Molybdenum Compounds

Molybdenum Compounds 109

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There is a disorder of the µ-H and µ-X atoms in the alkali metal salts that prevented the identification of the hydrogen atom in Rb3Mo2X8H and Cs3Mo2X8H by crystallographic means. However, the pyridinium salt (pyH)3Mo2Cl8H, which can be prepared by the usual method, exhibits no disorder problem thereby permitting its structure solution,316,317 including the location of the bridging hydrogen atom. With a Mo–Mo distance of 2.371(1) Å, a value which is similar to those in Rb3Mo2Cl8H (2.38(1) Å)312 and Cs3Mo2Br8H (2.439(7) Å),313 the presence of a fairly strong Mo–Mo bond is evident. The terminal Mo–Cl bonds trans to µ-H are significantly longer (by 0.10 Å) than those trans to µ-Cl. Refinement of the µ-H atom gave a Mo–H distance of c. 1.7 Å. These complexes bear a close structural relationship to the nonahalodimolybdate(III) anions except for the substantially shorter Mo–Mo distance in [Mo2Cl8H]3− compared to [Mo2Cl9]3− (by c. 0.28 Å). Subsequently, similar structures were determined for the [Mo2Cl8H]3− anion in the salts (Et4N)2(H5O2)[Mo2Cl8H],317 (Et4N)3(H5O2)- [Mo2Cl8H][MoOCl4(H2O)],298 and (Me4N)3Mo2Cl8H,318all of which have been prepared by the addition of R4NCl to solutions of Mo2(O2CCH3)4 in hot 12 M HCl. The structure determination of (Me4N)3Mo2Cl8H was carried out with the use of both X-ray and neutron diffraction methods.318 The Mo–Mo and Mo–H bond lengths were determined by neutron diffraction to be 2.357(3) Å and 1.823[7] Å, respectively. The bromo and iodo complexes (Me4N)2(H7O3)- [Mo2Br8H]319 and (Et4N)2(H7O3)[Mo2I8H]320 have been obtained by analogous procedures to these, and both salts structurally characterized. The Mo-Mo distances are 2.384(4) Å and 2.408(2) Å, respectively, and although neither anion is disordered the µ-H ligands were not located.

Various other compounds that contain the [Mo2Cl8H]3− anion have been prepared by oxidation of Mo2(O2CCH3)4, including (Bu4N)+, (4-MepyH)+, piperdinium, 8-hydroxyquino- linium and phenanthrolinium salts.321,322 The magnetic and spectroscopic properties and thermal characteristics of these compounds have been measured.321,322 The phosphonium salts (R3PH)3Mo2Cl8H (R = Et or Prn), have been prepared323 by the treatment of Mo2(mhp)4 with gaseous HCl and R3P in ethanol. These species show a resonance at β −3.7 (R = Et) and β −3.6 (R = Prn) in their 1H NMR spectra (recorded in CD3CN) that is assignable to the µ-H ligand.323 While the mixed metal carboxylate MoW(O2CCMe3)4 is not converted into [MoWCl8]4− upon treatment with hydrochloric acid, Katovic and McCarley324,325 have prepared Cs3MoWCl8H, a complex that is isostructural with Rb3Mo2Cl8H but whose Mo–W distance of 2.445(3) Å is longer than the Mo–Mo distance in Rb3Mo2Cl8H. This metal–metal bond lengthening which occurs upon formation of the heteronuclear dimer is in contrast to the bond shortening in the carboxylate dimer MoW(O2CCMe3)4 compared to Mo2(O2CCMe3)4. A detailed comparison has been made of the vibrational spectra of [Mo2Cl8H]3− and [MoWCl8H]3− and symmetric and asymmetric ι(M–H–M) modes assigned.325

4.3.3 Other aspects of dimolybdenum halogen compounds

A variety of studies that have focused upon the interrelationships between [Mo2X8]4−, [Mo2X8H]3− and the closely related [Mo2X9]3− species. A cyclic voltammetric study of the electrochemical oxidation of K4Mo2Cl8 in 6 M HCl has shown326 that a single oxidation wave is present at +0.5 V (versus SCE) with a shape very close to that expected for a reversible process. However, except at high sweep rates (500 mVs−1) the corresponding reduction peak was absent. For solutions of [Mo2Cl8]4− in the nonaqueous, basic AlCl3-ImCl melt system (ImCl = 1-methyl-3-ethylimidazolium chloride), two one-electron oxidations have been measured. With a glassy carbon electrode, these are at E1/2 5 −0.31 V and Ep,a c. +0.3 V.327 The first (reversible) oxidation generates [Mo2Cl8]3−; the second (irreversible) oxidation gives [Mo2Cl9]3−. When protonic impurities are present in these melts the [Mo2Cl8H]3− anion is generated. This

110Multiple Bonds Between Metal Atoms Chapter 4

problem can be circumvented by the addition of EtAlCl2, to the melt.328 This gives cleaner electrochemistry, with E1/2 = −0.16 V and Ep,a in the range 0.3 to 0.4 V (the value depending upon sweep rate) with the use of a Pt working electrode. A comparison of these data with the electrochemical properties of Mo2Cl4(PR3)4 compounds (Section 4.3.4) shows that the oxidation of [Mo2Cl8]4− to [Mo2Cl8]3− is much more cathodic than that of Mo2Cl4(PR3)4 to [Mo2Cl4(PR3)4]+. It has also been suggested327 that the oxidation observed at +0.5 V in the cyclic voltammogram of K4Mo2Cl8 in 6 M HCl326 is actually the irreversible second oxidation (i.e. [Mo2Cl8]3− Α [Mo2Cl9]3−), since the first oxidation should be overlapped by the H+/H2 redox couple in this medium, and therefore obscured.

A detailed study has been made of the redox chemistry interrelating [Mo2Cl8]4−, [Mo2Cl8H]3− and [Mo2Cl9]3− in the basic ambient temperature molten salt AlCl3–ImCl by employing electrochemistry and visible absorption spectroscopy.327,328 The electrochemical behavior of [Mo2Cl8H]3− in AlCl3–ImCl327 is quite different from that reported for solutions of [Mo2Cl8H]3− in CH2Cl2 and CH3CN.323

The kinetics of the oxidative addition of 6-12 M HCl to [Mo2Cl8]4− has been shown329 to be first order in [Mo2Cl8]4− and to obey a linear dependence with respect to the acidity function. The [Mo2Cl8H]3− anion decomposes in hydrochloric acid solutions (<3 M) to yield H2 and a hydroxy-bridged dimolybdenum(III) dimer. The 254 nm irradiation of [Mo2Cl8]4− in 3 M HCl has been found206 to produce [Mo2Cl8H]3−, probably through the reaction of H2O with a ligand-to-metal charge transfer excited state of [Mo2Cl8]4−. In a subsequent step, this anion decomposes thermally to yield 1 mole of hydrogen gas and the dimolybdenum(III) dimer [Mo2(µ-OH)2(aq)]4+. A similar reaction of [Mo2Br8]4− occurs in 3 M hydrobromic acid.206 No photoactivity was associated with irradiation of the β Α β* absorption band (located at 5 500 nm) of [Mo2X8]4−, an observation that was attributed206 to the persistence of strong metal–met- al bonding in low lying ββ* and β/* excited states.

While the oxidation of [Mo2Cl8]4− to [Mo2Cl8H]3− is clearly a quite facile process, the reversal of this reaction has been accomplished both in the basic AlCl3–ImCl melt system327,328 and in aqueous hydrohalic acid solutions. In the latter media, Bino and Gibson204 have shown that [Mo2X8H]3− and [Mo2X9]3− are reduced in a Jones reductor (amalgamated zinc) to afford a deep red solution containing Mo24+, from which K4Mo2Cl8·2H2O, Mo2(O2CCH3)4 and K4Mo2(SO4)4·2H2O can be crystallized upon addition of the appropriate anion. A similar conversion of [Mo2Cl9]3− and [Mo2Cl8H]3− to [Mo2Cl8l4− can be accomplished with the use of chromium(II) chloride in 6 M HCl.330

In addition to the reactions of Mo2(O2CCH3)4 with aqueous hydrohalic acids that afford halo-anions of dimolybdenum, two other reaction systems yield halide phases under different reaction conditions. These are:

1.reactions of solid Mo2(O2CCH3)4 with the gaseous hydrogen halides at elevated temperatures;

2.reactions with the gaseous hydrogen halides in non-aqueous media.

The first of these leads to phases of composition MoX2 as first demonstrated in the case of X = Cl. The brown powder that is formed upon reacting Mo2(O2CCH3)4 with dry gaseous HCl at 300 °C and which analyzes as molybdenum(II) chloride, has been designated as `-MoCl2.6,331,332 Much more recently, this halide has been prepared by the reaction between Mo2(O2CCH3)4 and AlCl3 in refluxing chlorobenzene.333 Its chemical reactions331-334 and spectroscopic properties333,335 show that this phase is not structurally related to _-MoCl2, in which a hexanuclear cluster of molybdenum atoms is present. However, there is evidence334,336 that heating the `-isomer at 350 °C leads to its conversion to _-MoCl2. The corresponding treatment of Mo2(O2CCH3)4 with hydrogen bromide and iodide at 300°C has been found to afford

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`-MoBr2 and `-MoI2.331,337 Another study of molybdenum bromide phases has also led to the isolation of a material purported to be `-MoBr2 (a dark green solid).338 However, the properties of this phase are not the same as those of `-MoBr2 that is prepared from Mo2(O2CCH3)4.331

In view of the ease of converting the `-MoX2 phases to complexes of the type Mo2X4L4 (L = pyridine or tertiary phosphine), it was originally suggested331,337 that they be formulated as [Mo2X4]n, i.e the parent halides of the [Mo2X8]4− anions. However, there is now evidence333 that these phases contain tetranuclear units, which would also react to form Mo2X4L4 molecules.

An interesting observation339 is that the thermal decomposition of Mo2Cl4(NHEt2)4 proceeds in four stages, whereby the NHEt2 is totally removed between 305 and 380 °C to give MoCl2. This may well be another route to `-MoCl2, but further study is necessary.

The reactions of Mo2(O2CCH3)4 with HX(g) in methanol lead, in all instances, to oxidation of the molybdenum and, in the presence of the appropriate Bun4N+ salts, to the crystallization of the salts (Bu4N)MoOCl4, (Bu4N)Mo2Br6 and (Bu4N)2Mo4I11.340 Thus, the extent of oxidation decreases in the order Cl > Br > I. (Bu4N)MoOCl4 is a well characterized MoV complex but (Bu4N)Mo2Br6 is of unknown structure, and therefore of uncertain nuclearity, although its chemical reactions have been interpreted340 in terms of the retention of a strongly bonded Mo–Mo unit. The paramagnetic iodide cluster (µeff = 1.95 BM and gav = 2.03 at room temperature)340 has been obtained by an alternative procedure that was devised by McCarley and co-workers.341 The tetranuclear structure of (Bu4N)2Mo4I11 has been confirmed by X-ray crystallography.341

A material of composition (Bu4N)2Mo2Br6 has been prepared342 from the reaction of Mo(CO)6 with Bu4NBr and dibromoethane in chlorobenzene. It is believed to be the one-electron reduced congener of (Bu4N)Mo2Br6. A study has been conducted on its reactions with monodentate and bidentate phosphine ligands,343 and both mononuclear and dinuclear complexes have been isolated (see Section 4.3.4).

4.3.4 M2X4L4 and Mo2X4(LL)2 compounds

In the majority of these compounds, X is a halogen (most often Cl), but others (e.g., NCS, NCO, R, OR, C>CR) also occur. The most common neutral ligands, L and LL, are monoand diphosphines, but more recently Mo2X4L4 molecules with L = amine have been prepared.

Starting materials that are most commonly used (but with many exceptions) are as follows:

1.A dimolybdenum halide that can itself easily be prepared from Mo2(O2CCH3)4, i.e. K4Mo2Cl8, (NH4)4Mo2Br8, (NH4)5Mo2Cl9·H2O, Cs3Mo2X8H (X = Cl or Br) or (picH)2[Mo2X6(H2O)2] (X = Br or I).

2.A mixture of Mo2(O2CCH3)4 or Mo2(O2CCF3)4 and Me3SiX (X = Cl, Br or I).

3.The dimolybdenum(II) carbonyl halides Mo2X4(CO)8; this is particularly important in the case of X = I.

4.A preformed complex of the type Mo2X4L4 (L is a monodentate ligand such as py or PR3) that is prepared by one of the three above methods and undergoes ligand exchanges (i.e., Mo2X4L4 + 4L' Α Mo2X4L'4 + 4L).

Table 4.7 lists a large number of (though not all) compounds and the starting materials from which they have been made. Table 4.8 lists the Mo2X4L4 compounds for which structural data are known, and Table 4.9 lists the Mo2X4(LL)2 compounds.

Table 4.7. Mo2X4L4 and Mo2X4(LL)2 compounds and the starting materials used in their synthesis

Compounda

Synthetic starting materials

Mo2X4(NH3)4; (X = Cl, Br or I)

Mo2X4(HNMe2)4; (X = Cl or Br)

Mo2Cl4(NMe3)4

Mo2X4(py)4; (X = Cl, Br or I)

Mo2X4(4-pic)4; (X = Cl, Br or I) Mo2X4(3-pic)4; (X = Cl or Br) Mo2X4(3,4-lut)4; (X = Cl or Br) Mo2X4(3,5-lut)4; (X = Cl or Br) Mo2X4(4-Butpy)4; (X = Cl or Br) Mo2Cl4(4-Butpy)4

Mo2Cl4(RNH2)4; (R = Et, Prn, But or Cy) [Mo2Cl4(pyz)2]n

Mo2Cl4(2,6-Me2pyz)4 Mo2X4(bpy)2

(X = Cl, Br or I) Mo2Cl4(phen)2

Mo2X4(NCR)4; (X = Cl, Br or I; R = Me, Et or Ph) Mo2Cl4(dpa)2

Mo2Cl4(amp)2

Mo2Cl4(8-aq)2 Mo2Cl4(Ph2Ppy)2

Mo2X4(PR3)4

(X = Cl, Br or I; PR3 = PMe3, PEt3, PPrn3, PBun3, PH2Ph, PMe2Ph, PEt2Ph, PHPh2, PMePh2 or PEtPh2) Mo2Cl4(PR3)4; (PR3 = PMe3, PMe2Ph or PHEt2) Mo2Cl4(PPh3)2(CH3OH)2

Mo2Cl4[P(OMe)3]4

Mo2X4(py)4 (X = Cl, Br or I),303,305 Mo2I4(4-pic)4305 MoX3 (X = Cl or Br)344,345

MoCl3346

Cs3Mo2X8H,347 (NH4)5Mo2Cl9·H2O,348 Mo2Cl4(dtdd)2,347 `-MoX2 (X = Cl, Br or I),331,337 Cs3Mo2Br7·2H2O,303 (picH)2[Mo2Br6(H2O)2],295 (Bu4N)Mo2Br6340 (NH4)5Mo2Cl9·H2O,349 (picH)2[Mo2X6(H2O)2] (X = Br or I)305,349

Cs3Mo2X8H350

Cs3Mo2X8H350

Cs3Mo2X8H350

Cs3Mo2X8H350

Mo2Cl6(THF)351

Mo2Cl6(THF)3352 (NH4)5Mo2Cl9·H2O350 (NH4)5Mo2Cl9·H2O350

(NH4)5Mo2Cl9·H2O350 Mo2Cl4(dtd)2,347 Mo2X4(py)4 (X = Br or I),305,347 Mo2I4(4-pic)4,305 (Bu4N)2Mo4I11340

Mo2Cl4(py)4348

Mo2Cl4(SMe2)4,347 [MoI2(THF)n]x (via Mo2I4(CO)8)353 (NH4)5Mo2Cl9·H2O354

(NH4)5Mo2Cl9·H2O354 (NH4)5Mo2Cl9·H2O354

K4Mo2Cl8,355 (NH4)5Mo2Cl9·H2O,355 Mo2Cl4(py)4,355 Mo2Cl4(PBun3)4,355 Mo2(O2CCH3)4/Me3SiCl126 K4Mo2Cl8,331,356 (NH4)5Mo2Cl9·H2O,347,357,358 Cs3Mo2Br8H,347 `-MoX2 (X = Cl, Br or I),331,337 Mo2Br4(py)4,347 (Bu4N)Mo2Br6,340 MoH4(PMePh2)4,359 Mo2I4(NCR)4 (via Mo2I4(CO)8),353 Mo2X4(CO)8 (X = Cl, Br or I),360-362 MoCl3(THF)3/Zn,21 Mo2(O2CCH3)4/Me3SiCl123,363

Mo2Cl4(NHEt2)4364 (NH4)5Mo2Cl9·H2O365 (NH4)5Mo2Cl9·H2O357

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		Atoms Metal Between

Compounda

Mo2Cl4[P(OMe)Ph2]4 Mo2Cl4(AsR3)4; (R = Me or Et) Mo2Br4(AsEt3)4 Mo2Cl4(dmpm)2 Mo2X4(dppm)2

(X = Cl, Br or I) _-Mo2Cl4(dmpe)2 `-Mo2X4(dmpe)2; (X = Cl or Br) `-Mo2Cl4(depe)2 _-Mo2Cl4(dedp)2 _-Mo2X4(dppe)2; (X = Cl or Br) `-Mo2X4(dppe)2

(X = Cl, Br or I) _-Mo2X4(dppee)2; (X = Cl or Br) `-Mo2X4(dppee)2; (X = Cl or Br) Mo2I4(dppee)2

_-Mo2Cl4(dpdt)2 `-Mo2Cl4(dpdt)2 _-Mo2Cl4(dpdbp)2 `-Mo2Cl4(dpdbp)2 _-Mo2Cl4(dptpe)2 _-Mo2Cl4(R-dppp)2

`-Mo2X4(S,S-dppp)2; (X = Cl or Br) _-Mo2X4(dppbe)2; (X = Cl or Br) `-Mo2Cl4[(R,R)-diop]2 `-Mo2Cl4[(S,S)-diop]2 _-Mo2Cl4(dppp)2

`-Mo2X4(dppp)2; (X = Cl or Br) Mo2Cl4(PPrn3)2(dppp)

Synthetic starting materials

(NH4)5Mo2Cl9·H2O358 K4Mo2Cl8,366 Cs4Mo2Cl8102 Cs3Mo2Br8H102

K4Mo2Cl8,367 Mo2(O2CCH3)4/Me3SiCl367

K4Mo2Cl8,368 Mo2X4(PEt3)4 (X = Cl or Br),368 Mo2(O3SMe)4,369 MoCl3(THF)3/Zn,21 Mo2(O2CCH3)4/Me3SiX (X = Cl, Br or I),123,370,371 Mo2I4(CO)8371

(NH4)5Mo2Cl9·H2O347 Mo2X4(PEt3)4 (X = Cl or Br)372,373

K2Mo2Cl8374

K2Mo2Cl8375

K2Mo2Cl8368,376 (NH4)4Mo2Br8,377 Mo2Cl4(py)4,376 Mo2(O2CCF3)4/Me3SiCl378

K4Mo2Cl8,376 Mo2X4(PEt3)4 (X = Cl or Br),368 Mo2Cl4(PBun3)4,368 Mo2Cl4(py)4,376 (Bu4N)Mo2Br6,340 Mo2(O2CCH3)4/Me3SiX (X = Cl or I),123,379 Mo2(O2CCF3)4/Me3SiX (X = Cl or Br)377,378 K4Mo2Cl8,380 (NH4)4Mo2Br8,380 Mo2(O2CCH3)4/Me3SiX380

K4Mo2Cl8,380 (NH4)4Mo2Br8,380 Mo2(O2CCH3)4/Me3SiX380 Mo2(O2CCH3)4/Me3SiI380

K4Mo2Cl8375

Mo2(O2CCF3)4/Me3SiCl375

K4Mo2Cl8381

Mo2(O2CCF3)4/Me3SiCl381

K4Mo2Cl8382,383 K4Mo2Cl8383

K4Mo2Cl8,384 Mo2(O2CCF3)4/Me3SiX384

K4Mo2Cl8,385 (NH4)5Mo2Cl9·H2O,385 (NH4)4Mo2Br8385

K4Mo2Cl8386

K4Mo2Cl8386

K4Mo2Cl8,376 Mo2Cl4(py)4376

K4Mo2Cl8,376 (NH4)5Mo2Cl9·H2O,387 Mo2Cl4(py)4,376 (NH4)4Mo2Br8387 Mo2Cl4(PPrn3)4376

Cotton		Compounds Molybdenum
	113

Compounda

Synthetic starting materials

`-Mo2X4(dppp)2; (X = Cl or Br) `-Mo2Cl4(tdpm)2 `-Mo2Cl4(S,S-bppm)2 `-Mo2X4(arphos)2b; (X = Cl or Br) Mo2Cl4(dpae)2

Mo2Cl4(diars)2 Mo2X4(DMF)4; (X = Cl or Br)

Mo2X4(SR2)4; (X = Cl or Br; R = Me or Et) Mo2Cl4(dth)2

Mo2Cl4(dto)2

Mo2X4(dtd)2 (X = Cl or Br) Mo2Cl4(dtdd)2

K4Mo2Cl8,388 Mo2(O2CCF3)4/Me3SiBr388 (NH4)5Mo2Cl9·H2O370

K4Mo2Cl8389

K4Mo2Cl8,368 (Bu4N)Mo2Br6340

K4Mo2Cl8368

K4Mo2Cl8368

Mo2Cl4(dtdd)2,347 Mo2Br4(SMe2)4347 (NH4)5Mo2Cl9·H2O,347 Mo2Br4(py)4347 (NH4)5Mo2Cl9·H2O347 (NH4)5Mo2Cl9·H2O390 (NH4)5Mo2Cl9·H2O347 Mo2Br4(DMF)4347

(NH4)5Mo2Cl9·H2O347

a The prefixes _ and ` signify different isomeric forms. These structural differences are discussed in the text.

b For X = Cl a mixture of _- and `-isomers is formed when (NH4)5Mo2Cl9·H2O is used as the synthetic starting material (see ref. 376).

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Molybdenum Compounds 115

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Table 4.8. Structures of Mo2X4L4 compoundsa,b
		Compound			Crystal	Virtual	Mo–Mo,	Twist			ref.
					Sym.	Sym.	Å	Angle (°)
				A. L = Phosphine
Mo2F4(PMe3)4					¯	D2d	2.110(5)	5	0		391
					43m
Mo2Cl4(PMe3)4					2	D2d	2.130(1)	0			356
					1	D2d	2.131(1)	~0			392
[Mo2I2(PBun3)2]2(µ-I)4					222	D2h	2.129[3]	50			393
Mo2(C>CH)4(PMe3)4					2	D2d	2.134(1)	NR			394
Mo2(C>CCH3)4(PMe3)4					1	D2d	2.141(1)	1.5			395
							2.140(1)
Mo2(C>CCMe3)4(PMe3)4					1	D2d	2.132(3)	~0			395
Mo2(C>CPri)4(PMe3)4					1	D2d	2.10(1)	?			396
Mo2(C>CSiMe3)4(PMe3)4					m	D2d	2.136(1)	1			395
Mo2Cl4(PHEt2)4					2	D2d	2.137(1)	2			364
Mo2Cl4(δ1-dmpm)4					222	D2d	2.137(1)	~0			364
Mo2Cl4	[	(Ph2P)2py	]	2	¯	C2h	2.149(1)	0			397
					1
Mo2Cl4[(NCCH2CH2)3P]2(MeCN)2·2MeCN					2	C2v	2.143(1)	~0			398
Mo2Cl4[(NCCH2CH2)3P]2(EtCN)2					m2m	C2v	2.139(2)	0			398
Mo2Cl4[(NCCH2CH2)3P]2(PriCN)2·PriCN					1	C2v	2.146(1)	3			398
Mo2Br4(PMe3)4					2	D2d	2.125(1)	50			366
Mo2I4(PMe3)4					1	D2d	2.127(1)	50			366
Mo2I4(PMe3)4·2THF					2	D2d	2.129(1)	50			360,362
Mo2Cl4(PEt3)4					¯	D2d	2.141(9)	0			358
					43m
Mo2Cl4(PMe2Ph)4					2	D2d	2.129(1)	50			358
Mo2Cl4(PMePh2)4·C6H6					1	D2d	2.135(1)	50			363
Mo2Cl4(PHPh2)4					1	D2d	2.147(1)	50			358
Mo2Cl4(PPh3)2(CH3OH)2					¯	C2h	2.143(1)	0			365
					1
Mo2(NCO)4(PMe3)4					2	D2d	2.134(1)	50			399
Mo2(NCS)4(PMe3)4					2	D2d	2.134(1)	1.5			399
Mo2(CH3)4(PMe3)4					2	D2d	2.153(1)	50			400
Mo2Cl4(PNP)(PHCy2)					1	Cs	2.147(1)	50			401
				B. L = Nitrogen atom donors
1,3,6,8-Mo2Cl4(NHEt2)4					2	D2d	2.133(1)	8.7(1)			339
Mo2Cl4(NH2Prn)4					222	D2d	2.118(2)	7.7			352
Mo2Cl4(NH2But)4					1	D2d	2.131(1)	3.9			352
							2.134(1)	NR
Mo2Cl4(NH2Cy)4					222	D2d	2.117(1)	2.8			352
Mo2Cl4[S-NH2(1-cyclohexylethyl)]4					222	D2d	2.127(4)	~6			402
Mo2Cl4[R-NH2(1-cyclohexylethyl)]4					222	D2d	2.121(4)	~6			402
Mo2Cl4(4-pic)4·CHCl3					¯	D2h	2.143(6)	0			351	a
					1
Mo2Br4(4-pic)4					¯	D2h	2.150(2)	0			349
					1
1,3,6,8-Mo2Cl4(4-pic)4					1	D2d	2.150(1)	9			351
1,3,5,7-Mo2Cl4(3,5-lut)4					1	D2h	2.142(1)	2			351
1,3,6,8-Mo2Cl4(3,5-lut)4					2	D2d	2.139(1)	9			351

116Multiple Bonds Between Metal Atoms Chapter 4

			Compound		Crystal	Virtual	Mo–Mo,		Twist	ref.
					Sym.	Sym.	Å		Angle (°)
		t	py)4·C6H5		¯	D2h	2.142(1)		0	351
Mo2Cl4(4-Bu					1
		t	2	/3THF	¯	D2h	2.140(1)		0	351
Mo2Cl4(4-Bu			py)4·		1
					1	D2d	2.138(1)		5	351
Mo2Cl4(4-Butpy)4·3/4C6H14					1	D2	2.141(1)		8	351
Mo2Cl4(4-Butpy)4·4/3CH2Cl2					1	D2	2.136(2)		14	351
					1	D2	2.150(2)		19
Mo2Cl4(4-Butpy)4·C6H6					2	D2	2.157(1)		22	351
Mo2Cl4(4-Butpy)4·acetone					1	D2	2.147(1)		22	403
		t	py)4·2C6H6		¯	D2h	2.148(2)		0	404
Mo2Br4(4-Bu					1
Mo2Cl4(NH2Prn)2(PMe3)2					2	C2v	2.125(1)		NR	405
Mo2Cl4(NH2Cy)2(PMe3)2					1	C2v	2.129(1)		NR	405
Mo2Cl4(NH2Cy)2(PMe2Ph)2					1	C2v	2.128(1)		NR	405
					C. X = alkoxide
Mo2(OPri)4py4					1	D2d	2.195(1)		NR	406
Mo2(OCH2CMe3)4(NHMe2)4					m	D2d	2.133(3)		NR	406
Mo2(OPri)4(HOPri)4					4	D2d	2.110(3)		~0	407,406
Mo2(O-c-Pen)4(HO-c-Pen)4					1	D2d	2.113(3)		NR	406
Mo2(OCH2CMe3)4(PMe3)4					1	D2d	2.218(2)		~0	407,406
Mo2(OCH2CMe3)4(HNMe2)4					1	D2d	2.133(2)		~0	407
Mo2(OC6F5)4(PMe3)4					¯	C2h	2.146(2)		0	408
					1
Mo2(OC6F5)4(HNMe2)4					2	D2d	2.140(2)		~0	409
				D. Miscellaneous structures
Mo2Cl4(SEt2)4					1	D2d	2.144(1)		~0	410
Mo2Cl2(HBpz)2					1	C2	2.155(1)		27	411
Mo2Br2(HBpz)2					1	C2	2.156(1)		28	411
Mo2I4(NCPh)4					2	D2d	2.144(5)		4	353
Mo2(	δ1	-O2CCF3)4(bpy)2			¯	C2h	2.129(1)	c	0	55
					1
Mo2(µ-CH2SiMe2CH2)(CH2SiMe3)2(PMe3)3					1	Ss	2.164(1)		d	412

a	When more than one crystallographically independent molecule is present, all independent Mo–Mo distances
	and ρ angles are listed.
b The idealized symmetry of the central Mo2L8 core.
c	The distance given in the reference (2.077 Å) is in error, although the structure is otherwise correct.
d This has MoI and MoIII atoms with 4 and 3 Mo–L bonds, respectively.
Table 4.9. Structures of Mo2X4(LL)2			compounds		with LL =		diphosphine or polyphosphine
	Compound	a		Crystal	Virtual		Mo–Mo,	Twist	ref.
				Sym.	Sym.b		Å	Angle (°)
	Mo2Cl4(dmpm)2			¯	C2h		2.125(1)	0	367
				1
	Mo2Cl4(dmpm)2·1/2H2O·11/3CH3OH			4/mmm	C2h		2.134(4)	0	367
	Mo2Cl4(dmpm)			2	D2h		2.127(1)	50
	Mo2Br4(dmpm)2			¯	D2h		2.127(1)	zero	413
				1
	Mo2Cl4(dmdppm)2			1	C2		2.152(1)	18	414
	Mo2I4(dmpm)2			1	D2		2.132(2)	11	413

Molybdenum Compounds 117

Cotton

			Compound		a	Crystal	Virtual	Mo–Mo,	Twist	ref.
						Sym.	Sym.b	Å	Angle (°)
Mo2Cl4(dippm)2						1	D2	2.170(1)	30	415
Mo2Cl4(dppm)2·2(CH3)2CO						¯	C2h	2.138(1)	0	369
						1
Mo2Cl4(dppm)·2CH2Cl2						1	D2h	2.150(1)	NR	364
Mo2(NCS)4(dppm)2·2(CH3)3CO						1	C2	2.167(3)	13.3	369
Mo2Br4(dppm)2·2THF						¯	C2h	2.138(1)	0	370
						1
Mo2Cl4(tdpm)2·2CH2Cl2						1	C2	2.148(1)	20	370
Mo2I4(dppm)2·2C7H8						1	C2h	2.139(1)	50	371
Mo2I4(dppm)2						¯	C2h	2.178(3)	0	416
						1
						1	C2	2.152(2)	17
`-Mo2Cl4(dmpe)2						2	D2	2.183(3)	40.0	372
`'-Mo2Cl4(dmpe)2c						D2	D2	2.168(1)	33.8	373
`-Mo2Br4(dmpe)2						1	D2	2.169(2)	36.5	373
`-Mo2Cl4(depe)2						D2	D2	2.173(2)	43.7	374
_	-Mo2Cl4(dppe)2·THF					¯	C2h	2.140(2)	0	417
						1
`-Mo2Cl4(dppe)2						1	D2	2.183(3)	30.5	378
`-Mo2Br4(dppe)2						1	D2	2.177(8)	31.1	418
`			2	/3CH2Cl2		¯	C2h	2.129(5)	0	379
	-Mo2I4(dppe)2·					1
						1	D2	2.180(4)	27.9
`-Mo2I4(dppe)2·C7H8						1	D2	2.179(3)	25.7	379
`-Mo2Cl4(dppee)2						1	D2	2.163(2)	25.5	380
anti-		_	-Mo2Cl4(dpdt)2·2CH3OH			¯	C2h	2.147(1)	0	375
						1
anti-		_	-Mo2Cl4(dpdbp)2			¯	C2h	2.149(1)	0	381
						1
`-Mo2Cl4(S,S-dppb)2·THF						1	D2	2.147(3)	24	384
`-Mo2Cl4(S,S-dppb)2·4CH3CN						1	D2	2.144(2)	22	384
`-Mo2Br4(S,S-dppb)2						1	D2	2.147(6)	21.7d	384
						1	D2	2.152(6)
`-Mo2Cl4(dpcp)2·0.5THF						2	D2	2.159(2)	522	419
`-Mo2Br4(dpcp)2·0.5THF						2	D2	2.155(4)	522	419
`-Mo2I4(dpcp)2·THF						2	D2	2.151(3)	522	419
`-Mo2Br4(arphos)2						1	C2	2.167(4)	30	420
`-Mo2Cl4(dppp)2						1	D2	2.156(3)	70.3	387
						1	D2	2.144(4)	68.5
`-Mo2Cl4[(R,R)-diop]2·3/4CH2Cl2						1	D2	2.149(1)	78	386
`-Mo2Cl4(S,S-bppm)2						1	C2h	2.128(2)	50	389
Mo2(OPri)4(dmpe)2						2	C2	2.236(1)	NR	421
Mo2(NCS)4(Ph2Ppy)2·2THF·2C7H8						2	C2	2.191(1)	11.0	422
Mo2Cl4(dppa)2						1	D2	2.134(1)	14	423
Mo2Br4(dppa)2·2THF						1	D2	2.137(1)	15	423
Mo2Cl4(dppa)2·2H2O						2	D2h	2.13(1)	23	145
Mo2Cl4(triphos)PEt3						1	C1	2.159(2)	12	424
Mo2Cl4(triphos)2						¯	Ci	2.149(6)	0	424
						1
meso-Mo2Cl4(tetraphos-1)						1	C1	2.186(1)	31	425

118Multiple Bonds Between Metal Atoms Chapter 4

	Compound	a	Crystal	Virtual	Mo–Mo,	Twist	ref.
			Sym.	Sym.b	Å	Angle (°)
	meso-Mo2Br4(tetraphos-1)·CH2Cl2		1	C1	2.195(3)	31	425
					2.183(3)	31	425
	meso-Mo2Br4(tetraphos-1)·1.5THF		1	C1	2.195(1)	29	425
	rac-Mo2Cl4(tetraphos-1)·CH2Cl2		2	C2	2.155(1)	18	426,425
	rac-Mo2Br4(tetraphos-1)·0.5CH2Cl2		2	C2	2.152(1)	19	425
	rac-Mo2Cl4(PEt3)(δ3-tetraphos-2)·C6H6		1	C1	2.132(3)	12	427
	_-Mo2Cl4[1,2-bis(2,5-dimethylphospholene)-		1	C2h	2.147(1)	5	428
	benzene]2·CH2Cl2
	Mo2(NCS)4(dppb)2·CH3NO2		2	D2	2.172(3)	26	429
					2.154(3)	22
	Mo2Cl4(bdppp)2·2CH2Cl2		¯	C2h	2.149(1)	0	397
			1
	trans-Mo2Cl4(2-Ph2P-6-Cl-py)2		¯	C2h	2.136(3)	zero	157
			1
	trans-Mo2Cl4(Ph2PCH2CO2Me)2		¯	C2h	2.145(1)	zero	430
			1
a	When more than one crystallographically independent molecule is present, all independent Mo–Mo distances
	and ] angles are listed.
b The idealized symmetry of the Mo24+ and its eight equatorial ligand atoms.
c	The prime signifies a different crystal form.

d Average P–Mo–Mo–P torsion angle for two independent molecules.

The first report of halide complexes of the type Mo2X4L4 was that of San Filippo,357 who isolated the phosphine complexes Mo2Cl4(PR3)4, where PR3 = PEt3, PPrn3, PBun3 or PMe2Ph, and the phosphite analog Mo2Cl4[P(OMe)3]4, upon reacting (NH4)5Mo2Cl9·H2O with the appropriate ligand in methanol under oxygen-free conditions. An interesting feature which was discovered in the 1H NMR spectra of Mo2Cl4(PR3)4 (and incidentally, in the related spectra of Re2Cl6(PR3)2)357 is the substantial deshielding of the ligand _-methylene protons as a consequence of the diamagnetic anisotropy associated with the M–M multiple bonds. Similar effects have subsequently been seen (Section 16.1.7) in the NMR spectra of other complexes that contain multiple bonds. In a later paper, San Filippo et al.347 reported a more extensive series of complexes of the type Mo2X4L4 (X = Cl or Br) which were prepared both from reactions of monodentate or bidentate ligands with (NH4)5Mo2Cl9·H2O or Cs3Mo2X8H, and via ligand exchange reactions from other preformed Mo2X4L4 complexes. Use of the latter method included the preparation of the acetonitrile and benzonitrile complexes Mo2Cl4(NCR)4 from Mo2Cl4(SMe2)4, and the conversion of the pyridine complex Mo2Br4(py)4 to Mo2Br4(SMe2)4, Mo2Br4(bpy)2, and Mo2Br4(PBun3)4. Mo2Cl4(SMe2)4 and Mo2Br4(py)4 were in turn prepared from (NH4)5Mo2Cl9·H2O and Cs3Mo2Br8H, respetively.347 With the use of these procedures, San Filippo et al.347 were able to establish the existence of such complexes with a variety of nitrogen, sulfur, and phosphorus donors plus the dimethylformamide complex Mo2Cl4(DMF)4.

Following this early work,347,357 a large number of complexes that contain monodentate

(L) or bidentate (LL) ligands have been prepared. In a few instances, complexes of these types have been generated in solution only, e.g., Mo2Cl4(PR3)4, where PR3 = P(OCH2CH2Cl)3, P(OCH2)3CEt, PClPh2 and P(CH=CH2)3.357 While the best strategies for preparing these complexes usually involve the use of well-defined dimolybdenum(III) or dimolybdenum(II) starting materials, other procedures exist that are of interest and significance in their own right even though they may not be the synthetic method of choice. Examples include the conversion of the methylsulfonate complex Mo2(O3SCH3)4 (prepared from Mo2(O2CCH3)4)212 to Mo2Cl4(dppm)2