Molecular structure of fac-[Mo(CO)3(DMSO)3]

The title compound fac-[Mo(CO)3(DMSO)3] was synthesized and crystallized as an alternative starting material for the more frequently used fac-[Mo(CH3CN)3(CO)3]. The DMSO complex was first reported in 1959 and apparently never mentioned again afterwards. Its crystal structure remained unknown to date. The observed metrical parameters of the now presented structural X-ray diffraction analysis are correlated to FT–IR data and compared to related complexes. Packing patterns and intermolecular non-classical hydrogen-bonding interactions are discussed.

The title compound, tricarbonyltris(dimethyl sulfoxide)molybdenum, [Mo(C 2 H 6 OS) 3 (CO) 3 ] or fac-[Mo(CO) 3 (DMSO) 3 ], crystallizes in the triclinic space group P1 with two molecules in the unit cell. The geometry around the central molybdenum is slightly distorted octahedral and the facial isomer is found exclusively. The packing within the crystal is stabilized by threedimensional non-classical intermolecular hydrogen-bonding contacts between individual methyl substituents of dimethyl sulfoxide and the oxygen atoms of either another dimethyl sulfoxide or a carbonyl ligand on adjacent complex molecules. The observed bond lengths in the carbonyl ligands and between carbonyl carbon atoms and molybdenum are correlated to the observed FT-IR bands for the carbonyl stretches and compared to respective metrical parameters of related complexes.

Chemical context
[Mo(CO) 6 ] is a commercially available starting material that is easy to handle. It is, however, not particularly reactive. In order to facilitate quicker and/or more complete reactions, it can be activated by replacing some of the CO ligands by solvent ligands. This is often done with acetonitrile, which results in fac-[Mo(CH 3 CN) 3 (CO) 3 ] complexes. Other examples comprise, for instance, [Mo(CO) 5 (THF)] (THF = tetrahydrofurane), [Mo(CO) 4 (nbd)] (nbd = norbornadiene) and fac-[Mo(CO) 3 (DMF) 3 ] (DMF = dimethyl formamide) (Wieland & van Eldik, 1991;Mukerjee et al. 1988;Villanueva et al., 1996). Depending on the co-ligand, the stability and reactivity of the resultant complex can be fine-tuned. It was, for example, previously emphasized that the pyridine complexes surpass acetonitrile complexes in reactivity (Kuhl et al., 2000). In cases where the carbonyl ligands are supposed to be retained, stronger carbonyl-metal interactions and very weak metal-co-ligand interactions are preferred. In cases where the carbonyl ligands shall also be replaced, the opposite is true. The grade of activation is reflected in the C O bond lengths and the Mo-C carbonyl bond lengths. For the former, infrared spectroscopy provides an easy way to probe the strength of the bond between carbon and oxygen with stretching vibration bands in a normally not populated region of the infrared wavenumber range (around 2000 cm À1 ). This bond strength depends directly on the metal-carbon interaction as the stronger the metal carbon bond, the weaker the carbon-oxygen bond becomes (Elschenbroich, 2003) and these again depend on the strengths of the trans-located coligand-to-metal interactions. A short and strong C O bond is, hence, indicative of only weak carbonyl metal-ligand interactions and concomitantly impaired complex stability. FT-IR therefore constitutes a particularly helpful assessment tool, in particular in cases where no crystal structure is available. On the other hand, it is also quite useful to combine both methods, if possible, for validation purposes and adding reliability to future spectroscopic evaluation of related species. In the course of synthesizing molybdenum-carbonyl complexes as starting materials and in a search for the optimum balance between reactivity and stability, various solvent complexes were tested in our group. During these experiments, DMSO was considered beneficial and the title complex fac-[Mo(CO) 3 (DMSO) 3 ] was prepared and crystallized. This complex was first reported in the literature in 1959 (Hieber et al., 1959), but its crystal structure remained, apparently, elusive to date. Notably, it also appears that since then the complex has never been mentioned again. As very nice and suitable crystals of the title compound were obtained, an X-ray diffraction structural analysis was carried out. The respective high-quality results, along with the signatory carbonyl FT-IR stretch bands are presented here.

Structural commentary
fac-[Mo(CO) 3 (DMSO) 3 ] crystallizes in the triclinic space group P1. The asymmetric unit represents the entire molecule ( Fig. 1) while Z = 2. The central zero-valent molybdenum is coordinated in a facial fashion by three neutral dimethyl sulfoxide and three neutral carbonyl ligands, i.e. it is embraced by a C 3 O 3 donor set. The coordination geometry of the complex is essentially octahedral, showing an almost perfect Bailar twist angle (Wentworth, 1972) of 59.08 . The average cis-donor-Mo-donor angle between the three coordinated DMSO molecules is, at approximately 79 , slightly more acute compared to that of the carbonyl ligands, at approximately 84 despite dimethyl sulfoxide being considerably more bulky. The three trans angles across molybdenum range from 173.76 (16) to 178.08 (18) , indicating a slight distortion from ideal octahedral geometry.
The structures of the title compound and those of chemically very closely related fac-[Mo(CH 3 CN) 3 (CO) 3 ] (refcode: IZUQAV; Antonini et al., 2004) and fac-[Mo(CO) 3 (DMF) 3 ] (refcode: WAJWIN; Pasquali et al., 1992) are, as expected, quite similar in the immediate coordination sphere surrounding molybdenum, which is also evident from the overlaid molecular structures (Fig. 2). Still, some specifics in the metrical parameter details in the individual species are quite notable.   two C-O bands are reported, and for the other two complexes, three. In perfectly octahedral symmetry, only two bands would be expected (Elschenbroich, 2003). The presence of three bands therefore indicates a distortion of the complex from perfect symmetry. The comparison of the highest energy infrared bands with the shortest observed C-O bond lengths in these three species reveals a perfect correlation (Fig. 3).
The O-donor solvents, therefore, appear to be slightly better suited for those reactions in which the carbonyl ligands are supposed to be retained on the metal, while the co-ligands are more labile. In the case of fac-[Mo(CO) 3 (DMSO) 3 ], it was observed that the complex is very sensitive to moisture, for instance, which supports the anticipated facile exchange of the coordinated solvents.
When larger co-ligands are also included in the C-O and Mo-C bond-length analysis, these observations are generally confirmed (Fig. 4). Only the structures with methyl-pyridine (refcode: TEMYOZ; Schut et al., 1996)

Figure 4
Average C-O and Mo-C distances in the molecular structures of neutral bisleptic triscarbonyl molybdenum(0) complexes with oxygen or nitrogen donor co-ligands. Hydrogen bonding between unit cells is exclusively between DMSO and carbonyl oxygen atoms (Fig. 5). The orientations of the molecules strictly alternate in the c-axis direction, as is evident when viewed along the ab diagonal (Fig. 6) while they are identical to those of their neighbours in the a-and b-axis directions.

Database survey
A search of the CSD database with ConQuest (Bruno et al., 2002) for bisleptic triscarbonyl molybdenum(0) complexes and three neutral co-ligands with N, O, S or P donor atoms results, in addition to the eight known molecular structures with oxygen or nitrogen donors, which are already discussed in the structural commentary, only in species with phosphorous donor atoms. These are structures with the refcodes DUSHAA , DUSHAA10 , JEWPIL (Nakazawa et al., 2006), KETQIJ (Campbell et al., 1999), KOBSIE (Fukumoto & Nakazawa, 2008), LALSEW (Willey et al., 1993), NIPTAH and NIPTEL (Alyea et al., 1997), NITFOM (Tallis et al., 2008), SANMOJ (Bent et al., 1989), SANMOJ10 (Bent et al., 1990), TAWNIO (Edwards et al., 1996), TIRYUP (Thirupathi et al., 2007), YAZSAT and YAZSIB (Kang et al., 1994), YAZSAT10 and YAZSIB10 (Hockless et al., 1996), YEPWAR (Fischer et al., 1994), and ZEXCIO (Alyea et al., 1995). The average C-O bond lengths in the molecular structures with phosphorous donor atoms range from 1.141 Å (KOBSIE) to 1.228 Å (DUSHAA). This means that both shorter as well as longer bonds are observed in the P-donor species than in the O-and N-donor complexes. In total only 27 examples of complexes are found in the database that meet the search criteria. Considering the simplicity of the complexes this is a surprisingly small number.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms belong to methyl substituents. They were attached to their parent atom in calculated positions based on the presence of electron density (HFIX 137) and treated as riding with U iso (H) = 1.5 U eq (C). One reflection was omitted from the refinement as a   SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: XP (Sheldrick, 2008); software used to prepare material for publication: CIFTAB (Sheldrick, 2008).

tricarbonyltris(dimethyl sulfoxide)molybdenum
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )