ORI GIN AL PA PER

Linkage Isomerism in [Mo3(l3-S)(l2-SSe)3(dtp)3]Cl:Preparation and Characterization of Two Isomerswith Different Coordination Mode of the l2-SSe Ligand

Rita Hernandez-Molina • Artem Gushchin •

Cristian Vicent • Pedro Gili

Received: 20 February 2014

� Springer Science+Business Media New York 2014

Abstract Two geometrical isomers of the composition [Mo3S4Se3(dtp)3]Cl (dtp is

O,O0-diethyldithiophosphate) have been prepared and characterized by multinuclear

NMR, electrospray ionization mass spectrometry (ESI–MS) and quantum chemical

calculations. The first isomer was prepared by reaction of the aqua cluster ([Mo3(l3-

S)(l2-Se)3(H2O)9]4?) with a large excess of P4S10/EtOH in 2 M HCl. Its structure

corresponds to [Mo3(l3-S)(l2-Seax-Seq)3(dtp)3]Cl (1). The second isomer, [Mo3(l3-

S)(l2-Sax-Seeq)3(dtp)3]Cl (2) was prepared by the reaction of [Mo3S7Cl6]2- with

SePPh3 followed by treatment with a stoichiometric amount of P4S10/EtOH. The reason

for the isomerism is that the l2-SSe ligand is coordinated asymmetrically, so that one

chalcogen atom is almost coplanar with the Mo3 plane (called equatorial) and the other

one is strongly out of the plane (axial). Alternative occupancy of these positions by S

or Se leads to a rare kind of linkage isomerism. Each isomer has distinctive spectro-

scopic signature and fragmentation pattern in ESI–MS. Isomer 2 with l2-SaxSeeq

R. Hernandez-Molina (&) � P. Gili

Departamento de Quımica Inorganica, 38200 La Laguna, Tenerife

e-mail: rrhernan@ull.es

R. Hernandez-Molina

Instituto Universitario de Quımica Biorganica, La Laguna, Tenerife

A. Gushchin

Nikolaev Institute of Inorganic Chemistry of the Russian Academy of Sciences, Novosibirsk, Russia

A. Gushchin

Novosibirsk State University, 630090 Novosibirsk, Russia

C. Vicent

Serveis Centrals d’Instrumentacio Cientıfica, Universitat Jaume I, Avda. Sos Baynat,

Castellon de la Plana, Spain

123

J Clust Sci

DOI 10.1007/s10876-014-0720-6

coordination mode was found to be slightly more stable than isomer 1 with l2-SeaxSeq

coordination.

Keywords Molybdenum clusters � Chalcogenide clusters � Linkage

isomerism � Dithiophosphates � Crystal structure

Introduction

The problem of selective synthesis of cluster complexes with two different

chalcogenide (Q = S, Se, Te) ligands in the cluster core remains a daunting task. In

the case of cluster cores with chalcogen atoms in structurally different bridging

positions, the selectivity can be achieved either during the cluster self-assembly,

when two different chalcogens go into their specific positions [1, 2], or by selective

chalcogen exchange in the specific positions, due to the difference in the reactivity

of structurally nonequivalent chalcogen atoms [3, 4].

An additional interest lies in the possibility of design of molecular conductors

using multiple Q–Q interactions in the solid state, based on chalcogenide cluster

complexes with easily accessible {Mo3(l3-Q)(l2-Qax-Qeq)3}4? core. For example, a

complex with 1,3-dithiol-2-thione-4,5-dithiolate (dmit), [Mo3S7(dmit)3], is a

semiconductor with an activation energy of 12–22 meV, and its structure features

formation of columns of the [Mo3S7(dmit)3] molecules by means of l3-S–l2-S2

interactions [5, 6]. Replacement of sulfur by selenium in [Mo3S7(dmit)3]2-

increases intermolecular Q–Q interactions in the solid state to afford layers in

[Mo3Se7(dmit)3]2- [7]. The bis(ethylenedithio)tetrathiafulvalene charge transfer

salts of [Mo3(l3-S)(l2-S2)3Cl6]2- and [Mo3(l3-S)(l2-SaxSeeq)3Br6]2- exhibit prop-

erties ranging from semiconducting to insulating, which in turn are determined by

various chalcogen–chalcogen (Q–Q) and chalcogen–halogen (Q–X) interactions in

the inorganic and organic sublattices [8]. It is obvious that, in order to better

understand the individual contributions of each type of interaction into the

conductivity path and to be able to optimize conducting behavior, ways of

preparation of the {Mo3(l3-Q)(l2-Q2)3}4? clusters with two different chalcogen

atoms in clearly defined positions must be found.

Earlier work has shown that these clusters exchange their bridging chalcogens

with SePPh3 and KNCSe [9, 10]. In this way, it was possible to obtain

heterochalcogenide complexes with the l2-SaxSeeq2- ligand, in which only equatorial

atoms were substituted, the {Mo3(l3-S)(l2-SaxSeeq)3}4? clusters. On the other hand,

sulfur addition to the monochalcogenide-bridged {Mo3(l3-Se)(l-Se)3}4? clusters

gives heterochalcogenide clusters with {Mo3(l3-Se)(l2-SeaxSeq)3}4? cores [11].

Later it was shown that high-temperature self-assembly reactions from the elements

in Mo–S–Se–Br system permitted isolation of new {Mo3(l3-S)(l2-Se2)3}4? and

{Mo3(l3-S)(l2-Se)3}4? clusters [1].

In this paper we report transformation of {Mo3(l3-S)(l2-Se)3}4? into new

{Mo3(l3-S)(l2-SeaxSeq)3}4? cluster which is related to {Mo3(l3-S)(l2-SaxSeeq)3}4?

as linkage isomer.

R. Hernandez-Molina et al.

123

Experimental

All the manipulations were done in air. The stock solution of [Mo3(l3-S)(l2-

Se)3(H2O)9]4? was prepared according to published procedures [1]. P4S10 was

purchased from ALDRICH and used without purification.

IR spectra were recorded in the 300–3,500 cm-1 range on a Perkin Elmer System

2000 FT-IR using KBr pellets. Characteristic IR frequencies for the dtp ligands were

assigned on the basis of previously reported complexes [12].

ESI Mass Spectrometry and CID Mass Spectra

A Q-TOF Premier mass spectrometer with an orthogonal Z-spray-electrospray

interface (Waters, Manchester, U.K.) was used. The desolvation gas as well as cone

gas was nitrogen at a flow of 400 and 30 L/h respectively. The temperature of the

source block was setto 120 �C and the desolvation temperature to 150 �C. Mass

calibration was performed using a solution of sodium iodide in isopropanol/water

(50:50) from m/z 100 to 1,900. A capillary voltage of 3.3 kV was used in the

positive scan mode, and the cone voltage was set to 10 V to control the extent of

fragmentation of the identified ions. Chloroform:acetonitrile (50:50) sample

solutions were infused via syringe pump directly connected to the ESI source at a

flow rate of 10 ll/min. For collision induced dissociation (CID) experiments, the

cations of interest were mass-selected using the first quadrupole (Q1) and interacted

with argon in the T-wave collision cell at variable collision energies

(CE = 0–40 eV). The complete envelope of each ion was mass-selected.

Preparation of [Mo3(l3-S)(l2-SeqSeax)3((EtO)2PS2)3]Cl (1)

To a 20 ml solution of [Mo3SSe3(H2O)9]4? in 2 M HCl (4 mM) a solution of 2 g

P4S10 (4.5 mmol) in 10 ml EtOH was added and a copious brown precipitate

appeared which was filtered and the recrystallized from chloroform to give orange

crystals by slow evaporation of the solvent. Yield: 70–80 %. Electrospray ionization

mass spectrometry (ESI–MS) (CHCl3/MeCN): m/z = 1210 ([Mo3S4Se3(dtp)3]?).31P NMR (CDCl3): d = 98.62 ppm. 77Se NMR (CDCl3): d = 131 ppm. IR (KBr,

4000–400 cm-1): 2977 m, 2933 w, 2896 w, 2863 w, 1471 m, 1440 m, 1386 m,

1288 w, 1160 m, 1102 w, 1054 s, 1004 s, 969 s, 815 s, 792 s, 638 s, 532 m,

436 m.

Preparation of [Mo3(l3-S)(l2-SaxSeeq)3((EtO)2PS2)3]Cl (2)

(TBA)2[Mo3S4Se3Cl6] (180 mg, 0.13 mmol) prepared from the reaction of

(TBA)2[Mo3S7Cl6] with SePPh3 as described in the literature [10] was dissolved

in CH2Cl2 (30 ml). P4S10 (45 mg, 0.10 mmol) in 5 ml of EtOH were added to the

solution of cluster. The mixture was refluxed for 2 h. The final solution was filtered

and evaporated to dryness under reduced pressure. The solid was washed with H2O,

i-PrOH and ether. Yield: 85 mg (50 %). ESI–MS (CHCl3/MeCN): m/z = 1210

([Mo3S4Se3(dtp)3]?). 31P NMR (CDCl3): d = 96.42 ppm. 77Se NMR (CDCl3):

Linkage Isomerism in [Mo3(l3-S)(l2-SSe)3(dtp)3]Cl

123

d = -107 ppm. IR (KBr, 4000–400 cm-1): 2958 m, 2931 m, 2896 w, 2869 w,

1470 m, 1440 m, 1384 m, 1286 w, 1159 m, 1098 w, 1028 s, 1006 s, 961 s, 812 s,

772 s, 638 m, 531 m, 458 w, 438 w.

Computational Methods

Ground-state electronic structures calculations have been performed with density

functional theory (DFT) methods using the Gaussian 03 software package [13]. The

hybrid density functional B3LYP was applied, which consists of the non-local

hybrid exchange functional as defined by Becke’s three parameters equation, and of

non-local Lee–Yang–Parr correlation functional [14, 15]. In all calculations the

LANL2DZ basis set was used which included f functions for the metal [16–18] and

6–31G(d, p) bases for non-metallic atoms H, O, S and Se. The LANL2DZ basis set

included Dunning/Huzinaga [19] valence double-n and effective-core potential

(ECP). The electronic ground-state was left to full geometry optimization without

symmetry constraints. All cores (charge 4 and singlet spin) were optimized in C1

symmetry and also with charge 1 and doblet spin (reduced species) in the same

symmetry for cations 1 and 2. For the cations with charge 4 the BNO analysis was

carried out in accordance with Ref. [20].

Results and Discussion

Synthesis and Characterization

Isomeric cluster complexes 1 and 2 were prepared in the reaction sequence shown in

Fig. 1. Preparation of 1 involved reaction of [Mo3(l3-S)(l2-Se)3(H2O)9]4? with a

freshly prepared solution of P4S10 in ethanol. Earlier it was shown that [Mo3(l3-Se)

(l2-Se)3(H2O)9]4? under similar conditions yielded [Mo3(l3-Se)(l2-SeaxSeq)3

(dtp)3]Cl [11]. In these preparations P4S10 acts both as dtp source, according to

the well-known reaction:

P4S10 þ 8 EtOH ! 4 ðEtOÞ2PðSÞSH þ 2 H2S;

and as source of ‘‘active’’ sulfur which converts l2-Se into l2-SSe. Most probably,

the source of this ‘‘active’’ sulfur is a mixture of tri- and tetrasulfide,

(EtO)2P(S)SnP(S)(OEt)2, which also is formed from P4S10 and EtOH [21].

Complex 2 was prepared by reaction of [Mo3S7Cl6]2- with SePPh3 as a source of

selenium, which selectively gives [Mo3(l3-S)(l2-SaxSeeq)3Cl6]2-, followed by

chloride substitution upon treatment with a stoichiometric amount of P4S10 in

ethanol. Complexes 1 and 2 are air stable and soluble in polar organic solvents.

Their purity and individuality was confirmed by 77Se and 31P NMR and by ESI mass

spectrometry.

The ESI mass spectra of chloroform:acetonitrile (50:50) solutions of compounds

1 and 2 were identical where the base peak was the pseudo-molecular

[Mo3S4Se3(dtp)3]? cation centered at m/z 1210.2. However, both isomers could

be readily differentiated on the basis of their characteristic fragmentation reactions

R. Hernandez-Molina et al.

123

upon CID conditions. For Mo3Q74? clusters, gas-phase selective elimination of

diatomic ‘‘Q2’’ from equatorial positions can be used as a diagnostic of the identity

of the chalcogen in the equatorial position [22, 23]. Figure 2 illustrates the ESI mass

spectrum of [Mo3(l3-S)(l2-SaxSeeq)3((EtO)2PS2)3]Cl (2) and CID spectrum of

mass-selected [Mo3(l3-S)(l2-SaxSeeq)3((EtO)2PS2)3]? cation at m/z 1210 where the

loss of diatomic Se2 is clearly observed, thus confirming the Se occupation at the

equatorial sites. Analogously, CID mass spectrum of mass-selected [Mo3(l3-S)(l2-

SeaxSeq)3((EtO)2PS2)3]? cation at m/z 1210 displayed elimination of S2 under

identical conditions.

Coordinated dtp ligands in 1 and 2 show single lines in 31P NMR at 98.62 ppm

(1) and 96.42 ppm (2) which is typical for coordinated dtp ligands. More important

is difference in the chemical shifts of Se atoms: for l2-SeaxSeq (1) d is 131 ppm,

while for l2-SaxSeeq (2) d is -107 ppm, thus Se in equatorial position is more

shielded that Se in axial position. If this relationship applies also to the

asymmetrically coordinated l2-Se2 ligands in the complexes with Mo3Se74? core,

it becomes possible to finally assign most shielded peaks (with negative chemical

shifts) to Se atoms in the equatorial positions. Thus for [Mo3Se7(C2O4)3]2-,

[Mo3Se7(N(SePPh2)2)3]?, and [Mo3Se7Br6]2- the following pair of peaks can be

assigned within l2-Se2: to Seax 287.1, 210, and 286.6 ppm, to Seeq -85.8, -97 and

-133.7 ppm, respectively [1, 24, 25].

DFT Calculations

To find out the relative stability of the linkage isomers we have carried out DFT

calculations on the [Mo3(l3-X)(Qeq-Qax)3(H2O)6]? cations (10: X = S, Qeq = S,

Mo

SMo Mo

SS

S

S

SSClCl

Cl

ClCl

Cl

2-

SePPh3

Mo

SMo Mo

SS

S

Se

SeSeClCl

Cl

ClCl

Cl

2-

P4S10/EtOH

Mo

SMo Mo

SS

S

Se

SeSeSS

S

SS

S

+

dtp

[Mo3S7Cl6]2- [Mo3S4Se(eq)3Cl6]2- [Mo3S4Se(eq)3(dtp)3]+ (2)

Mo

SMo Mo

SeSe

Se

Se

SeSeBrBr

Br

BrBr

Br

2-

PPh3

Mo

SMo Mo

SeSe

Se

OH2H2O

OH2

OH2H2O

H2O

4+

P4S10/EtOH

Mo

SMo Mo

SeSe

Se

S

SSSS

S

SS

S

+

dtp

H2O

OH2H2O

[Mo3SSe6Br6]2- [Mo3SSe3(H2O)9]4+ [Mo3S4Se(ax)3(dtp)3]+ (1)

H2O

Fig. 1 General scheme of preparation of [Mo3S4Se3(dtp)3]? isomers

Linkage Isomerism in [Mo3(l3-S)(l2-SSe)3(dtp)3]Cl

123

Qax = Se; 20: X = S, Qeq = Se, Qax = S; 3: X = Se, Qeq = Se, Qax = S; 4:

X = Se, Qeq = S, Qax = Se). We have optimised the structures of the isomers 10

and 20 containing the l3-S cap and those of the isomers 3 and 4 containing the l3-Se

cap. The energies for 10, 20, 3 and 4 are: -2.480517, -2.480518, -3.005715 and

-3.005713 9 10-7 kJ/mol, respectively (see Table 1). It is concluded that for each

family of isomers always the isomer with Se in the equatorial position is slightly

more stable. The eigenvalues for HOMO and LUMO orbitals are: -1.963, -1.609

for 10; -1.965, -1.611 for 20; -1.964, -1.611 for 3 and -1.968, -1.612 9

10-3 kJ/mol for 4.

In Table 1 it is shown the main geometrical parameters of the calculated

complexes. The calculated bond distances calculated for the aqua clusters are in

good agreement with those found in the literature for related clusters. The average

(a)

(b)

Fig. 2 a ESI mass spectrum of [Mo3(l3-S)(l2-SaxSeeq)3((EtO)2PS2)3]Cl (2) and b CID spectrum ofmass-selected [Mo3(l3-S)(l2-SaxSeeq)3((EtO)2PS2)3]? cation at m/z = 1,210 at increasing collisionenergies CElaboratory = 5 (bottom), 15 (middle) and 25 (top) eV

R. Hernandez-Molina et al.

123

Mo–Mo distances for the clusters 10 and 20 having a l3-S is of 2.64 A whereas the

average Mo–Mo bond distance for compounds 3 and 4 having a l3-Se ligand is of

2.78 A average. In the reported compound [Mo3(l3-Se)(l-SeaxSeq)3((EtO)2PS2)3]Cl

[11] having a l3-Se atom the Mo–Mo distance is of 2.74 A, which is in good

agreement with the calculated values. The Mo–l3Q distance is of 2.40 A average for

compounds 10 and 20 while for compounds 3 and 4 is of 2.51 A. In the structure of

[Mo3(l3-Se)(l-SeaxSeq)3((EtO)2PS2)3]Cl [11], the reported value for Mo–l3Se is of

2.51 A identical to the calculated value for compounds 3 and 4. It is clearly seen that

the incorporation of Se due to its larger atomic radii causes an enlargement in the

bond distance. The atom that occupies the equatorial position is bounded to the

metal more loosely than the atom that occupies the axial position as it is indicated

by the larger bond distances associated to the bond Mo–Qeq. In our computational

calculations we have found the same tendency in the Mo–Q bond distances (see

Table 1). The S–Se bond distances are of 2.289, 2.263, 2.201, 2.195 A,

respectively for compounds 10, 20, 3 and 4, being slightly larger for the compounds

with the l3-S ligands. In the reported structures of [Mo3Se4S3((EtO)2PS2)3]Cl [11],

[Mo3S4Se3(dtc)3]SeCN [26] and [Mo3S4Se3(dtc)3]Br [27] the S–Se bond distances

range between 2.16 and 2.24 A which are in good agreement with the calculated

values. In these comparisons it should be considered that the terminal ligands are

different. For the calculated structures water is the terminal ligand while for the

other compounds the terminal ligand is dithiophosphate or dithiocarbamate.

Conclusions

A synthetic procedure has been developed to prepare the two linkage isomers

[Mo3(l3-S)(l2-Seax-Seq)3(dtp)3]Cl (1) and[Mo3(l3-S)(l2-Sax-Seeq)3(dtp)3]Cl (2).

The isomers have been characterized by spectroscopy. DFT calculations gave

geometrical parameters in good agreement with those reported in the literature for

related compounds.

Table 1 The data of DFT calculations

Complex Mo–

MoaMo–

l3(Q)

Mo–

l2(Q)eq

Mo–

l2(Q)ax

Mo–

O

S–Se Eb HOMOc LUMOc gd

10 2,614 2,402 2,502 2,582 2,271 2,289 -2,4805 -1,963 -1,609 0.177

20 2,667 2,404 2,66 2,462 2,276 2,263 -2,4805 -1,965 -1,611 0.176

3 2,781 2,509 2,66 2,462 2,189 2,201 -3,0057 -1,964 -1,611 0.1765

4 2,775 2,507 2,555 2,571 2,188 2,195 -3,0057 -1,968 -1,612 0.178

10 = [Mo3(l3-S)(Seq-Seax)3(H2O)6]4?; 20 = [Mo3(l3-S)(Seeq-Sax)3(H2O)6]4?; 3 = [Mo3(l3-Se)(Seeq-

Sax)3(H2O)6]4?; 4 = [Mo3(l3-Se)(Seq-Seax)3(H2O)6]4?

a Distances in Angstromsb E 9 10-7 kJ mol-1

c 910-3kJ mol-1

d g = ELUMO-EHOMO/2

Linkage Isomerism in [Mo3(l3-S)(l2-SSe)3(dtp)3]Cl

123

Acknowledgments Rita Hernandez-Molina acknowledges the Spanish Ministery of Science and

Innovation for fundings through Projets Estudio cinetico-mecanısticos de reacciones inorganicas

mediante la obtencion de parametros de activacion: aproximacion integral CTQ12-37821-C02-01 and

Diseno y Sıntesis de nuevas moleculas moduladoras de dianas terapeuticas a traves de estructuras

privilegiadas y sıntesis orientada a la diversidad SAF2012-37344-C03-01. A. Gushchin thanks the

Russian Foundation for Basic Research (Grants 12-03-00305 a, and 12-03-33028 a).

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