Inorganica Chimica Acta 394 (2013) 289–294 Contents lists available at SciVerse ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica Phosphinoarylthiolato molybdenum and iron complexes [M{(SC6H4-2-PPh2)-j2S,P}2 (CO)2] (M = Mo, Fe): Analogous composition – Different structure Ana-Maria Vălean a,b, Santiago Gómez-Ruiz b,c, Alexandru Lupan a, Radu Silaghi-Dumitrescu a, Luminita Silaghi-Dumitrescu a,⇑, Evamarie Hey-Hawkins b,⇑ a b c Faculty of Chemistry and Chemical Engineering, ‘‘Babes-Bolyai’’ University, M. Kogălniceanu 1, RO-400084 Cluj-Napoca, Romania Institut für Anorganische Chemie der Universität Leipzig, Johannisallee 29, D-04103 Leipzig, Germany Departamento de Química Inorgánica y Analítica, Universidad Rey Juan Carlos, C/ Tulipán s/n, E-28933 Móstoles (Madrid), Spain a r t i c l e i n f o Article history: Received 11 March 2012 Received in revised form 12 May 2012 Accepted 31 May 2012 Available online 26 July 2012 Keywords: Phosphinoarylthiolato-molybdenum complex Phosphinoarylthiolato-iron complex Trigonal prism cis–trans geometries Stereochemical isomers a b s t r a c t The molecular structures of the phosphinoarylthiolato molybdenum and iron complexes [Mo{(SC6H4-2PPh2)-j2S,P}2(CO)2] (1) and [Fe{(SC6H4-2-PPh2)-j2S,P}2(CO)2] (2) are reported. They both have a central hexacoordinate metal(II) cation, but compound 1 has a trigonal-prismatic geometry with both CO and both thiolato groups in cis arrangement, while iron(II) complex 2 displays a pseudo-octahedral structure with both CO, both phosphino and both thiolato groups in trans arrangement. This is in a good agreement with DFT calculations, which showed that the different coordination geometry of molybdenum and iron in complexes 1 and 2 is a consequence of the number of d electrons (electronic factor) combined with the geometrical factors. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction 2. Experimental Hemilabile ligands containing one strongly binding and one weakly binding donor group are useful ligands in catalytically active complexes [1], but can also be used for bridging two different metals. Complexes of the type Ga{(SC6H4-2-PPh2)-j2S,P}{(SC6H4-2PPh2)-jS}2, Ga(tBu){(SC6H4-2-PPh2)-j2S,P}{(SC6H4-2-PPh2)-jS} or Ga(tBu){(SC6H4-2-AsPh2)-j2S,As}{(SC6H4-2-AsPh2)-jS} [2] in which one or two ligands are bound solely by the sulfur atom leaving one pendant phosphino or arsino group might be potential ligands for coordination to a second metal (e.g., Mo, Cr, Fe, etc.). With this in mind, the reaction of Ga{(SC6H4-2-PPh2)-j2S,P}{(SC6H4-2-PPh2)-jS}2 with molybdenum or iron carbonyl complexes was performed, but only the compounds [Mo{(SC6H42-PPh2)-j2S,P}2(CO)2] (1) and [Fe{(SC6H4-2-PPh2)-j2S,P}2(CO)2] (2) were obtained as the result of a transmetalation/redox reaction. The molecular structures of 1 and 2 are reported here. 2.1. General procedures ⇑ Corresponding authors. Tel.: +40 264 593833; fax: +40 264 590818 (L. SilaghiDumitrescu), tel.: +49 (0) 341 9736151; fax: +49 (0) 341 9739319 (E. HeyHawkins). E-mail addresses: lusi@chem.ubbcluj.ro (L. Silaghi-Dumitrescu), hey@uni-leipzig.de (E. Hey-Hawkins). 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2012.05.041 All manipulations were carried out under an inert atmosphere of dry nitrogen. Ga{(SC6H4-2-PPh2)-j2S,P}{(SC6H4-2-PPh2)-jS}2 [2], Mg{(SC6H4-2-PPh2)-j2S,P}2 [3], [Mo(CO)4(nbd)] (nbd = norbornadiene, g4-C7H8) [4] and fac-[Mo(CH3CN)3(CO)3] [5] were obtained according to literature procedures. Toluene, bis(2methoxyethyl) ether (diglyme), diethyl ether and tetrahydrofuran (THF) were dried over sodium–benzophenone, distilled under an atmosphere of dry argon and stored over potassium mirror. C6D6 was dried over sodium–potassium alloy, filtered and kept under an inert atmosphere over potassium mirror. 1 H and 31P NMR spectra were recorded on a Bruker Avance DRX-400 instrument. 1H NMR: internal standard TMS. 31P NMR: external standard 85% H3PO4. The infrared spectra were recorded on a Perkin-Elmer System 2000 FT-IR spectrometer scanning between 4000 and 400 cm1 by using KBr pellets (KBr dried in vacuum, 150 °C, 103 torr, 24 h). The samples were prepared in a glove box. The crystallographic data were collected on a CCD Oxford Xcalibur S diffractometer, Mo Ka radiation (k = 0.71073 Å), x- and u-scan mode. Data reduction was carried out with CRYSALISPRO including empirical absorption correction with SCALE3 ABSPACK [6]. The structure was elucidated by direct methods using 290 A.-M. Vălean et al. / Inorganica Chimica Acta 394 (2013) 289–294 SHELXS-97 and was refined using SHELXL-97 [7]. Non-hydrogen atoms were refined anisotropically, and H atoms were calculated on idealized positions. One-half of a very disordered toluene solvent molecule was found in the asymmetric unit of 1, and one-half of a very disordered THF solvent molecule in the asymmetric unit of 2; both were removed using the program SQUEEZE implemented in PLATON [8]. Structure figures were generated with ORTEP [9]. Thermal ellipsoids are drawn at 50% probability. Geometries were optimized with the M06-2X [10] functional as implemented in the GAUSSIAN software package [11] using the 631G basis set for all atoms except molybdenum, for which the SDD basis set was used, which combines DZ and the StuttgartDresden ECP relativistic effective core potential basis sets [12]. separated by hand and characterized. The colorless crystals proved to be the starting material (Ga{(SC6H4-2-PPh2)-j2S,P}{(SC6H4-2PPh2)-jS}2), whereas the yellow-orange crystals (ca. 60 mg) proved to be [Fe{(SC6H4-2-PPh2)-j2S,P}2(CO)2] (2). (b) In a similar procedure, a yellow-orange solution of [Fe(CO)5] (0.688 mmol, 0.09 ml 7.45 M in THF) was added dropwise at 20 °C to a colorless solution of Mg{(SC6H4-2-PPh2)-j2S,P}2 (0.210 g, 0.344 mmol) in THF (10 ml). The yellow solution was stirred at room temperature for 3 h and then concentrated in vacuo to one-quarter of its volume. Very few yellow-orange crystals of 2 were obtained from this concentrated THF solution after a few days at room temperature. 2.2. Synthesis of [Mo{(SC6H4-2-PPh2)-j2S,P}2(CO)2] (1) 3. Results and discussion (a) A yellow solution of [Mo(CH3CN)3(CO)3] (0.0712 g, 0.235 mmol) in 10 ml of THF was added dropwise to a solution of Ga{(SC6H4-2-PPh2)-j2S,P}{(SC6H4-2-PPh2)-jS}2 (0.223 g, 0.235 mmol) in 10 ml THF, and the reaction mixture was stirred for 37 h at room temperature. The resulting dark redbrown solution was concentrated in vacuo until a red-violet solid precipitated. The red-violet precipitate was characterized by 1H, 31 P NMR and IR spectroscopy. 1H NMR (d, C6D6, ppm): 6.6– 7.6 ppm (H, aryl-H). 31P {1H} NMR (d, C6D6, ppm): 80.6 (s) and 74.6 (s). IR (KBr, cm1): 1940 cm1 (s) and 1858 (s) (CO stretching vibration). Attempts to further purify the product by recrystallization were unsuccessful. A few red crystals of [Mo{(SC6H4-2-PPh2)j2S,P}2(CO)2] (ca. 15 mg) were obtained from different solvents: THF, diglyme, Et2O, etc. (b) A few red crystals of 1 were also obtained from the 1:1 reaction of Ga{(SC6H4-2-PPh2)-j2S,P}{(SC6H4-2-PPh2)-jS}2 (0.45 g, 0.474 mmol) with [Mo(CO)4(nbd)] (0.14 g, 0.486 mmol) in toluene. The 1:1 reactions of Ga{(SC6H4-2-PPh2)-j2S,P}{(SC6H4-2-PPh2)jS}2 with fac-[Mo(CH3CN)3(CO)3], [Mo(CO)4(nbd)] and [Fe(CO)5] resulted in a transmetalation and redox reaction yielding the metal(II) complexes [Mo{(SC6H4-2-PPh2)-j2S,P}2(CO)2] (1) and [Fe{(SC6H4-2-PPh2)-j2S,P}2(CO)2] (2) in very low yield as red-violet (1) or yellow-orange (2) crystals (Scheme 1). All attempts to isolate any reduction products and to establish the oxidation–reduction processes failed. As magnesium chalcogenolates can also act as transmetalation reagents, Mg{(SC6H4-2-PPh2)-j2S,P}2 [3] was reacted with [Fe(CO)5], but unexpectedly only compound 2 was obtained in very low yield. In this case it is unclear which compound was reduced. Iron carbonyl thiolato complexes with tertiary phosphine ligands have been intensively studied [13], with emphasis on their structural features, their possible applications in catalysis as well as the closely related biological relevance to the active site of the Fe-only hydrogenases [14]. In this context [Fe{(SC6H4-2-PPh2)j2S,P}2(CO)2] (2) was already reported [15] and was obtained by treatment of HSC6H4-2-PPh2 with an acetonitrile solution of FeCl2 saturated with CO. The IR spectrum of 2 shows an intense, sharp band at 1979 cm1 due to mCO [15]. The IR spectrum of [Mo{(SC6H4-2-PPh2)-j2S,P}2(CO)2] (1) shows two strong absorptions in the carbonyl region (mCO) at 1940 and 1858 cm1. The 1H NMR spectrum of 1 is not very helpful, since it shows only several multiplets between 6.6 and 7.6 ppm for the phenyl groups. The 31P{1H} NMR (C6D6) spectrum shows two singlets at 80.6 and 74.6 ppm of equal intensity. The computed 31P NMR spectrum of 1 at the M06–2X/6-31G/SDD level shows two 2.3. Synthesis of [Fe{(SC6H4-2-PPh2)-j2S,P}2(CO)2] (2) (a) A yellow-orange solution of [Fe(CO)5] (0.432 mmol, 0.06 ml 7.45 M in THF) was added dropwise at 0 °C to a solution of Ga{(SC6H4-2-PPh2)-j2S,P}{(SC6H4-2-PPh2)-jS}2 (0.410 g, 0.432 mmol) in 8 ml of THF. The reaction mixture was then stirred at room temperature for 1 h. The obtained yellow solution was concentrated in vacuo to less than half its volume and stored at room temperature for crystallization. Over 2 days, a mixture of colorless and yellow-orange crystals was obtained. The crystals were PPh2 Ph2P + fac -[Mo(CH 3 CN)3 (CO) 3] S or [Mo(CO) 4(nbd)] 1:1 S Mo S CO CO PPh2 S Ga P Ph2 S (1) Ph2 P + [Fe(CO) 5] 1:1 Ph2 P S S CO Ph2 P Fe OC + [Fe(CO) 5] PPh 2 Mg 1:2 S P Ph2 (2) Scheme 1. Synthetic routes to 1 and 2. S 291 A.-M. Vălean et al. / Inorganica Chimica Acta 394 (2013) 289–294 Table 2 Summary of data collection, structure solution and refinement details for 1 and 2. Compound 1 2 Empirical formula C38H28MoO2P2S2 0.5toluene 784.67 220(2) triclinic  P1 C38H28FeO2P2S2THF FW T (K) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (Mg/m3) l (mm1) F(0 0 0) Crystal size (mm3) hMin/hMax (°) No. of reflections collected No. of independent reflections Completeness to hMax (%) Final R indices [I > 2r(I)] R indices (all data) Fig. 1. Molecular structure of [Mo{(SC6H4-2-PPh2)-j2S,P}2(CO)2] (1). Hydrogen atoms are omitted for clarity. slightly different signals for the two phosphorus atoms, supporting experimental data for inequivalent phosphorus atoms within a slightly distorted C2 molecular symmetry (see Supplementary information for more details). A few red crystals were obtained from different solvents (e.g., diethyl ether, THF or diglyme) at room temperature. The X-ray crystal structure analysis revealed the complex [Mo{(SC6H4-2PPh2)-j2S,P}2(CO)2] (1) (Fig. 1 and Table 1).  with two Complex 1 crystallizes in the triclinic space group P1 molecules in the unit cell (Table 2). Table 1 Selected bond lengths (Å) and bond angles (°) in compounds 1 and 2. 1 2 Mo(1)–C(1) Mo(1)–C(2) Mo(1)–S(1) Mo(1)–S(2) Mo(1)–P(1) Mo(1)–P(2) O(1)–C(1) O(2)–C(2) 1.946(4) 1.948(4) 2.403(1) 2.416(1) 2.429(1) 2.433(1) 1.171(4) 1.154(4) Fe(1)–C(19) Fe(1)–S(1) Fe(1)–P(1) C(19)–O(1) 1.808(5) 2.295(1) 2.231(1) 1.144(5) C(1)–Mo(1)–C(2) C(1)–Mo(1)–S(1) C(1)–Mo(1)–S(2) C(2)–Mo(1)–S(1) C(2)–Mo(1)–S(2) S(1)–Mo(1)–S(2) C(1)–Mo(1)–P(1) C(2)–Mo(1)–P(1) C(1)–Mo(1)–P(2) C(2)–Mo(1)–P(2) S(1)–Mo(1)–P(1) S(2)–Mo(1)–P(1) S(1)–Mo(1)–P(2) S(2)–Mo(1)–P(2) P(1)–Mo(1)–P(2) 108.0(1) 140.47(8) 94.5(1) 97.0(1) 142.5(1) 82.60(4) 80.06(9) 74.87(9) 75.14(9) 79.4(1) 77.53(3) 139.98(3) 140.91(3) 77.95(4) 136.34(3) C(19)–Fe(1)–C(190 ) C(19)–Fe(1)–S(1) C(19)–Fe (1)–S(10 ) S(1)–Fe(1)–S(10 ) C(19)–Fe(1)–P(1) C(190 )–Fe(1)–P(1) S(1)–Fe(1)–P(1) S(1)–Fe(1)–P(10 ) P(1)–Fe(1)–P(10 ) 180.0 93.4(1) 86.6(1) 180.0 89.2(1) 90.9(1) 87.27(4) 92.73(4) 180.0 Goodness-of-fit (GOF) on F2 Largest difference in peak (e Å3) 11.062(5) 13.024(5) 13.025(5) 98.310(5) 95.226(5) 95.226(5) 1838.7(13) 2 1.417 0.592 802 0.2  0.08  0.03 2.57/25.68 36 961 6976 [Rint = 0.0711] 99.9 R1 = 0.0328, wR2 = 0.0745 R1 = 0.0591, wR2 = 0.0827 0.924 0.565 and 0.353 770.62 130(2) monoclinic I2/a 18.889(2) 9.8572(8) 21.208(3) 90 112.07(1) 90 3659.3(7) 4 1.399 0.653 1600 0.1  0.05  0.02 2.85/25.35 26 849 3342 [Rint = 0.1036] 99.9 R1 = 0.0414, wR2 = 0.1105 R1 = 0.0886, wR2 = 0.1233 0.950 0.408 and 0.268 The molybdenum atom is hexacoordinated by two (SC6H4-2PPh2) ligands through sulfur and phosphorus atoms and two additional CO groups in a trigonal-prismatic geometry. The two carbonyl groups and the two sulfur atoms are in a cis orientation (C(1)–Mo(1)–C(2) 108.0(1)°, S(1)–Mo(1)–S(2) 82.60(4)°), while the P(1)–Mo(1)–P(2) bond angle is much larger (136.34(3)°). The Mo–S bond lengths (2.403(1) and 2.416(1) Å) are shorter than the reported Mo–S bonds found in neutral [Mo{(SC6H4-2)3 P-j4S,S0 ,S00 ,P}2] (2.438–2.626 Å) [16], thiolato-bridged dinuclear [Mo2(l-SC6H4-2-PPh2-j2S,P)3Cl3] (2.413–2.505 Å) [17] or anionic [NMe4][Mo{(SC6H4-2)2PPh)-j3S,S0 ,P}(CO)3] [18] phosphinoarylthiolato molybdenum complexes. However, they are longer than those found in [MoO(SCH2CH2PPh2-j2S,P)2] (2.372–2.348 Å) [19] or [Mo{(SCH2CH2)2PPh-j3S,S0 ,P}2] (2.340–2.355 Å) [20]. The Mo–P bond lengths (2.429(1) and 2.433(1) Å) are in the same range as those found in [Mo(CO)3(ETPB)3] (ETPB = 4-ethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane) (av. Mo–P 2.428(5) Å) [21] or in fac-[Mo(CO)3{P(OC6H5)3}3] (2.435(8) Å) [22] and slightly shorter than the corresponding bonds in the phosphinothiolato molybdenum complexes of P(SHC6H4-2)3 [16], [Mo{PhP(CH2CH2S)2j3S,S0 ,P}2] (2.466–2.478 Å) [20], [Mo(CO)4(MeSC6H4-2-PPh2j2S,P)] (2.498(1) Å), [Mo(CO)4{(MeSC6H4-2)2PPh-j2S,P}] (2.5223 (9) Å) [23], [Mo(CO)5(MeOC6H4-2-PPh2-jP)] (2.539(1) Å) and [Mo (CO)5{(MeOC6H4-2)2PPh-jP}] (2.5680(5) Å) [24]. The Mo(1)–C(1) and Mo(1)–C(2) bond lengths (1.946(4) and 1.948(4) Å) are similar to those found in [Mo(CO)4{PPh2 (CH2)2SMe2-j2S,P}]BF4 (Mo–C(O) (trans to S): 1.943(5) Å) and [Mo(CO)4{PPh2(CH2)2SMe-j2S,P}] (Mo–C(O) (trans to S): 1.957(4) Å) [25] and shorter than the Mo–C bonds in the binary carbonyl [Mo(CO)6] (2.059 Å) [26]. Strong intramolecular S(1)  S(2) interactions [27] of 3.180 Å were observed in complex 1; the distance between the two sulfur atoms is smaller than the sum of the van der Waals radii of the atoms involved (Rv. d. Waals radii (S  S) = 3.6 Å [28] or 3.8 Å, based on data derived from Me2S [29]). 292 A.-M. Vălean et al. / Inorganica Chimica Acta 394 (2013) 289–294 Table 3 DFT-derived relative energies (kcal/mol) for the experimentally-observed compounds 1 and 2 as well as for putative related structures and isomers. Fig. 2. Molecular structure of [Fe{(SC6H4-2-PPh2)-j2S,P}2(CO)2] (2). Hydrogen atoms are omitted for clarity. Yellow-orange crystals of 2 were obtained from a concentrated THF solution at room temperature in a few days. Compound 2 crystallizes in the monoclinic space group I2/a with four molecules in the unit cell (Table 2). It consists of discrete molecules in which the metal center is coordinated by two monoanionic bidentate (SC6H4-2-PPh2) ligands and two CO groups (Table 1 and Fig. 2). The geometry around the Fe atom can be described as slightly distorted octahedral with all dihedral angles between the coordination planes defined by four donor atoms being close to 90°; angles defined by two trans donor atoms and the metal are 180°, and those involving the iron center and two mutually cis donor atoms lie in the range of 86.6(1)–93.4(1)°. The structure is centrosymmetric (Fe(1) is located on a crystallographic inversion center) with trans-CO, trans(P,P) and trans(S,S), as predicted by the spectroscopic data [15]. The Fe–S (2.295(1) Å) and Fe–P (2.231(1) Å) bond lengths are unremarkable and close to the values found in [Fe{SC6H3-2-PPh26-SiMe3-j2S,P}3] [15], [Me3BzN]2[Fe(CN){(SC6H4-2)3P-j4S,S0 ,S00 ,P}] (MeOH) [30], [Fe(SPh)2(CO)2(dppe)] (dppe = Ph2PCH2CH2PPh2) [31], [R4N][Fe2(l-S2){(SC6H4-2)3P-j4S,S0 ,S00 ,P}2] [13], [Fe2(CO)6 (l-SPh)(l-PPh2)] [32], [Fe2(CO)5(PPh3)(l-SPh)(l-PPh2)] [13], [Fe2 (CO)6(l-SC6F5)(l-PPh2)] [13], the cluster [Fe4(CO)11 {P(OCH3)3}(SC6H4-2-CS2-j3S,S0 ,S00 )] [33], etc. Moreover, the Fe–S bond lengths of the two sulfur atoms in trans arrangement are in good agreement with Fe–Strans in [Fe2(CO)4(l-SR)2(l-dppm)] (R = Ph, p-tol, dppm = Ph2PCH2PPh2) and [Fe2(CO)4{l-S2CN (p-tol)}(l-dppm)] [34]. The Fe–C(O) bond (1.808(5) Å) is slightly longer than those found in similar iron carbonyl compounds, e.g. [Fe(quinoxaline-2,3-dithiolate)(CO)2(PMe3)2] (1.760(2) Å) [35] and [Fe2(S2C3H6-j2S,S0 )(CO)3(dppv)(PMe3)2(NO)]BF4 (dppv = cis1,2-bis(diphenylphosphino)ethene) (Fe–C 1.74(1)–1.75(1) Å) [13] and lies at the end of the range of Fe–C bond lengths (1.70– 1.80 Å) found for six-coordinate iron(II) carbonyl complexes [36]. The crystallographic data and refinement details for compounds 1 and 2 are shown in Table 2. 3.1. cis versus trans geometries in [M{(SC6H4-2-PPh2)-j2S,P}2(CO)2] (M = Mo, Fe) complexes For complexes 1 and 2, geometries were optimized using density functional theory. Additionally, models were constructed in each case, where the molybdenum in 1 was replaced by Fe and where the iron in 2 was replaced by Mo – thus providing for both 1 and 2 two isomers, denoted cis and trans based on the positions of the CO ligands. Additionally, versions of these complexes were Model cis trans 1 (Mo) 1, chloroform 1, H 1 – Mo(0),H [2] 2 (Fe) 2, H 2 – Fe(IV),H [+2] 0.00 0.00 0.00 0.00 +8.09 +3.16 0.00 +7.38 +9.78 +3.32 +23.10 0.00 0.00 +49.70 computed in which the phosphorus-bound phenyl groups were replaced by hydrogen (‘‘1, H’’ and ‘‘2, H’’ in Table 3), as well as versions where the d-electron configurations were changed by adjusting the overall charge on the model to yield an iron(IV) model (isoelectronic to the molybdenum(II) complex 1) and a molybdenum(0) model (isoelectronic to the iron(II) complex 2). The computed relative energies for the cis–trans isomers are shown in Table 3, and geometries are illustrated in the Supporting information. DFT predicts for the molybdenum complex 1 the cis isomer to be favored by 7 kcal/mol, while for the iron complex 2, the trans isomer is more stable by 8 kcal/mol – in good agreement with the fact that experimentally only the respective isomers were observed. The reasons for the different preferences of molybdenum and iron may involve d-electron configuration (two d electrons less in Mo compared to Fe), steric factors or solvation. The cis isomers, especially for molybdenum, appear to favor a trigonal-prismatic geometry as opposed to octahedral. An ideal octahedron would feature a value of 180° for the largest S–M–S or P–M–P angles, as opposed to 136° for an ideal trigonal prism [37]. The cis isomer of model 1 features a P–M–P angle of 135°, hence 100% trigonal-prismatic from this point of view (although, some distortions are inevitable due to the distinctly different lengths of the Mo–S, Mo–P and Mo–C bonds and to the constraints imposed by the bite angles of the bidentate ligands). By contrast, trans-1 has the same angles at 180° values, suggesting a 100% octahedron. For cis-2 (the iron complex), the largest angle is 172°, suggesting >85% octahedral character [37]. The d-electron configuration is expected to play an important role in controlling the coordination geometry. Indeed, Hoffmann et al. [38] pointed out that transition metal centers with fewer d electrons (e.g., molybdenum as opposed to iron) tend to favor alternatives to octahedral geometries, such as trigonal prisms. This would explain why (as detailed in the Supporting information) the distortion from octahedral geometry is indeed stronger in the cis isomer of the molybdenum model complex compared to the cis isomer of the iron model complex, and also why this distortion is distinctly larger in the cis isomer of the FeIV model complex compared to the cis isomer of the FeII model complex. Table 3 shows that for the molybdenum complex 4 kcal/mol of the 7 kcal/mol favoring the cis isomer (i.e., almost 60%) are lost when the phenyl substituents on phosphorus are replaced by hydrogen atoms (models cis/trans-Mo,H). Likewise, for the iron complex 2, 5 kcal/mol of the 8 kcal/mol energy difference between the isomers (60%) are lost when the phosphorus-bound phenyl groups are removed. Indeed, in the cis isomer of the Fe model complex, distances of 3.25 Å are seen between the sulfur atoms and the closest carbon atoms in the P-bound phenyl rings; these distances are 0.5 Å shorter than the sum of the respective van der Waals radii and thus likely to involve steric repulsion, disfavoring the cis isomer. By contrast, in the cis-Mo model complex there is no such sulfur–carbon distance; instead, the phenyl group near each sulfur atom has a hydrogen atom located 2.84 Å from the sulfur atom, A.-M. Vălean et al. / Inorganica Chimica Acta 394 (2013) 289–294 0.3 Å shorter than the sum of the van der Waals radii and most likely implying an attractive interaction. Interestingly, because of the larger radius of molybdenum, the attractive S  H(phenyl) interactions are no longer possible in the trans-molybdenum complex, where this distance is much larger than the sum of the van der Waals radii (4.15 Å). Solvation appears to have a modest but non-negligible effect on the isomer preference: for the molybdenum complex, the energy differences change by 2 kcal/mol in chloroform compared to vacuum. We, therefore, propose that the preferences of molybdenum and iron in complexes 1 and 2 for a cis versus a trans geometry arise from a combination of electronic and geometrical factors: on one hand the smaller number of d electrons controls the preference between octahedral and trigonal-prismatic geometries, while on the other hand attractive and repulsive non-covalent interactions within each isomer modulate these preferences. 4. Conclusions The phosphinoarylthiolato complexes [Mo{(SC6H4-2-PPh2)- j2S,P}2(CO)2] (1) and [Fe{(SC6H4-2-PPh2)-j2S,P}2(CO)2] (2) were obtained from Ga{(SC6H4-2-PPh2)-j2S,P}{(SC6H4-2-PPh2)-jS}2 and molybdenum or iron carbonyl complexes in a transmetalation/redox reaction. Complex 2 was also obtained in low yield from the reaction of Mg{(SC6H4-2-PPh2)-j2S,P}2 with [Fe(CO)5]. In both 1 and 2 the metal center is hexacoordinated by two monoanionic bidentate (SC6H4-2-PPh2) and two CO ligands. In compound 1, the geometry around the metal center corresponds to a trigonal prism, whereby the two carbonyl groups and the two sulfur atoms are in a cis orientation, while a centrosymmetric octahedral structure with trans-CO, trans-P,P and trans-S,S was observed for 2. This is in a good agreement with the DFT calculations, which showed that the different coordination geometry of molybdenum and iron in complexes 1 and 2 is a consequence of the number of d electrons (the electronic factor) combined with the geometrical factors. Acknowledgments This work was possible with the financial support of the Sectoral Operational Programme for Human Resources Development 2007–2013, co-financed by the European Social Fund, under the Project number POSDRU 89/1.5/S/60189 with the title ‘‘Postdoctoral Programs for Sustainable Development in a Knowledge Based Society’’ (A.M.V.). Appendix A. Supplementary material CCDC 870580 and 870581 contains the supplementary crystallographic data for complexes 1 and 2. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2012.05.041. 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