Connecting Main-Group Metals (Al, Ga, In) and Tungsten(0) Carbonyls via the N2S2 Metallo-Ligand Strategy

Tetradentate N2S2 ligands (such as bismercaptoethanediazacycloheptane in this study) have seen extensive use in combination with transition metals. Well-oriented N2S2 binding sites are ideal for d8 transition metals with square planar preferences, especially NiII, but also as a square pyramidal base for those metals with pentacoordinate preferences, such as [V≡O]2+, [Fe(NO)]2+, and [Co(NO)]2+. Further reactivity at the thiolate sulfurs generates diverse bi, tri, and tetra/heterometallic compounds. Few N2S2 ligands have been explored to investigate the possibility of binding to main group metals, especially group III (MIII) metals, and their utility as synthons for main group/transition metal bimetallic complexes. To open up this area of chemistry, we synthesized three new five-coordinate main group XMN2S2 complexes with methyl as the fifth binding ligand for M = Al, and chloride for M = Ga and In. The seven-membered diazacycle, dach, was engaged as a rigid stabilized connector between the terminal thiolate sulfurs. The pentacoordinate XMN2S2 complexes were characterized by 1H-NMR, 13C-NMR, +ESI-Mass spectra, and X-ray diffraction. Their stabilities and reactivities were probed by adding NiII sources and W(CO)5(THF). The former replaces the main group metals in all cases in the N2S2 coordination environment, demonstrating the weak coordinate bonds of MIII–N/S. The reaction of XMN2S2 (XM = ClGaIII or ClInIII) with the labile ligand W(0) complex W(CO)5(THF) resulted in Ga/In–W bimetallic complexes with a thiolate S-bridge. The synthesis of XMN2S2 complexes provide examples of MIII–S coordination, especially Al–S, which is relatively rare. The bimetallic Ga/In–S–W complex formation indicates that the nucleophilic ability of sulfur is retained in MIII–S–R, resulting in the ability of main group MIII–N2S2 complexes to serve as metalloligands.


Introduction
In the early 1960s, Busch and co-workers reported the complexing ability of mercaptamines (NS ligands) for Ni II [1].Since then, various NS ligands have been reported, including those with a contiguous S-N-N-S, N 2 S 2 , tetradentate donor set, an arrangement that mimics the N 2 S 2 coordination environment rendered by a cysteine-glycine/serine-cysteine tripeptide motif found at three enzyme active sites [2].Further development has been robust.Of special note is the bimetallic Ni-Ni site in acetyl co-A synthase, ACS, in which N 2 S 2 is viewed as a tight Ni-binding site, while the second nickel is labile and catalytically active in the C-C coupling reactions required of ACS, as shown in Figure 1 [3].We developed NiN2S2 complexes as metallodithiolate ligands, especially engaging diazamesocycles N2C6H8 (daco), N2C5H10 (dach), and N2C4H8 (dach*) as stabilizing units in N to N connections [4−6].Those containing the more flexible "open-chain" ethylene or propylene N to N linkers bring more flexibility to the N2S2 binding unit, resulting in subtle differences in reactivity and stability properties.The ability to tune the MN2S2 ligands by variations of M has resulted in a range of complexes, such as V 4+ in [V≡O] 2+ , Fe 2+ in {Fe(NO)} 7 , Ni 2+ , Pd 2+ , Cu 2+ ,and Zn 2+ [7].The ability of the MN2S2 metalloligands to serve as monodentate as well as bidentate ligands to a single metal or as bridging bidentate ligands to two metals has led to multiple new compositions and various diverse structural forms [7].A few of these are illustrated in Figure 2 [7].However, the chemistry of N2S2 derivatives of main group metals remains relatively unexplored, and their ability to serve as aggregation sites for exogeneous metals has until now, to our knowledge, been unreported.While the expectation that the soft S will have a poor binding affinity for hard Al(III) is reasonable, a Cambridge database search located 36 results of N2S2 derivatives of aluminum.Most of them are derived from an aluminum (I) precursor [8,9].The use of abundant aluminum in catalysis is well known, including as catalysts for CO2/epoxide cycloaddition [10,11], which comprises a tetradentate N2O2 ligand with an axial X ligand (X = alkyl, halogen completing the coordination sphere).In contrast, many more sulfur-coordinated gallium(III) and indium(III) complexes have been reported, owing to the softer core of Ga(III)/In(III) [12][13][14][15][16][17].Explorations in those cases have been prompted by the wide use of 111 In and 67 Ga as radionuclides in PET (Positron Emission Tomography) and SPECT (Single Photon Emission Computed Tomography) [18].We developed NiN 2 S 2 complexes as metallodithiolate ligands, especially engaging diazamesocycles N 2 C 6 H 8 (daco), N 2 C 5 H 10 (dach), and N 2 C 4 H 8 (dach*) as stabilizing units in N to N connections [4][5][6].Those containing the more flexible "open-chain" ethylene or propylene N to N linkers bring more flexibility to the N 2 S 2 binding unit, resulting in subtle differences in reactivity and stability properties.The ability to tune the MN 2 S 2 ligands by variations of M has resulted in a range of complexes, such as V 4+ in [V≡O] 2+ , Fe 2+ in {Fe(NO)} 7 , Ni 2+ , Pd 2+ , Cu 2+ ,and Zn 2+ [7].The ability of the MN 2 S 2 metalloligands to serve as monodentate as well as bidentate ligands to a single metal or as bridging bidentate ligands to two metals has led to multiple new compositions and various diverse structural forms [7].A few of these are illustrated in Figure 2 [7].However, the chemistry of N 2 S 2 derivatives of main group metals remains relatively unexplored, and their ability to serve as aggregation sites for exogeneous metals has until now, to our knowledge, been unreported.We developed NiN2S2 complexes as metallodithiolate ligands, especially engaging diazamesocycles N2C6H8 (daco), N2C5H10 (dach), and N2C4H8 (dach*) as stabilizing units in N to N connections [4−6].Those containing the more flexible "open-chain" ethylene or propylene N to N linkers bring more flexibility to the N2S2 binding unit, resulting in subtle differences in reactivity and stability properties.The ability to tune the MN2S2 ligands by variations of M has resulted in a range of complexes, such as V 4+ in [V≡O] 2+ , Fe 2+ in {Fe(NO)} 7 , Ni 2+ , Pd 2+ , Cu 2+ ,and Zn 2+ [7].The ability of the MN2S2 metalloligands to serve as monodentate as well as bidentate ligands to a single metal or as bridging bidentate ligands to two metals has led to multiple new compositions and various diverse structural forms [7].A few of these are illustrated in Figure 2 [7].However, the chemistry of N2S2 derivatives of main group metals remains relatively unexplored, and their ability to serve as aggregation sites for exogeneous metals has until now, to our knowledge, been unreported.While the expectation that the soft S will have a poor binding affinity for hard Al(III) is reasonable, a Cambridge database search located 36 results of N2S2 derivatives of aluminum.Most of them are derived from an aluminum (I) precursor [8,9].The use of abundant aluminum in catalysis is well known, including as catalysts for CO2/epoxide cycloaddition [10,11], which comprises a tetradentate N2O2 ligand with an axial X ligand (X = alkyl, halogen completing the coordination sphere).In contrast, many more sulfur-coordinated gallium(III) and indium(III) complexes have been reported, owing to the softer core of Ga(III)/In(III) [12][13][14][15][16][17].Explorations in those cases have been prompted by the wide use of 111 In and 67 Ga as radionuclides in PET (Positron Emission Tomography) and SPECT (Single Photon Emission Computed Tomography) [18].While the expectation that the soft S will have a poor binding affinity for hard Al(III) is reasonable, a Cambridge database search located 36 results of N 2 S 2 derivatives of aluminum.Most of them are derived from an aluminum (I) precursor [8,9].The use of abundant aluminum in catalysis is well known, including as catalysts for CO 2 /epoxide cycloaddition [10,11], which comprises a tetradentate N 2 O 2 ligand with an axial X ligand (X = alkyl, halogen completing the coordination sphere).In contrast, many more sulfur − coordinated gallium(III) and indium(III) complexes have been reported, owing to the softer core of Ga(III)/In(III) [12][13][14][15][16][17].Explorations in those cases have been prompted by the wide use of 111 In and 67 Ga as radionuclides in PET (Positron Emission Tomography) and SPECT (Single Photon Emission Computed Tomography) [18].
In order to extend the coordination chemistry of main group III metals with this versatile ligand motif, we explored the synthesis and characterization of Group III elements, such as Ga/In-Cl or Al-R, bound within the N 2 S 2 binding cavity.We probed the availability of the residual S lone pairs to serve as nucleophiles for the binding to exogeneous metals.In particular, heterobimetallics were formed with W(CO) 4/5 , a receiver unit with the CO reporter unit, which provides a reference point for establishing donor ability of such metalloligands [7].

Synthesis and Characterizations of XMN 2 S 2 Complexes
The H 2 bme-dach (N,N -Bis(Mercaptoethyl)-1,4-Diazacycloheptane) was synthesized according to a reported procedure [4].Under an N 2 atmosphere, a solution of AlMe 3 , GaCl 3 , or InCl 3 was transferred by a double-ended needle into a solution of H 2 bme-dach ligand in a Schlenk flask.White powdery products formed immediately, resulting in a white suspension.After stirring overnight, the solvent was removed under vacuum giving white solids, determined to be the compounds shown in Figure 3.They were further washed with Et 2 O and pentane, giving a yield of 75-80%.
Inorganics 2019, 7, x FOR PEER REVIEW 3 of 9 In order to extend the coordination chemistry of main group III metals with this versatile ligand motif, we explored the synthesis and characterization of Group III elements, such as Ga/In-Cl or Al-R, bound within the N2S2 binding cavity.We probed the availability of the residual S lone pairs to serve as nucleophiles for the binding to exogeneous metals.In particular, heterobimetallics were formed with W(CO)4/5, a receiver unit with the CO reporter unit, which provides a reference point for establishing donor ability of such metalloligands [7].

Synthesis and Characterizations of XMN2S2 Complexes
The H2bme-dach (N,N′-Bis(Mercaptoethyl)-1,4-Diazacycloheptane) was synthesized according to a reported procedure [4].Under an N2 atmosphere, a solution of AlMe3, GaCl3, or InCl3 was transferred by a double-ended needle into a solution of H2bme-dach ligand in a Schlenk flask.White powdery products formed immediately, resulting in a white suspension.After stirring overnight, the solvent was removed under vacuum giving white solids, determined to be the compounds shown in Figure 3.They were further washed with Et2O and pentane, giving a yield of 75-80%.The geometries of XMN2S2 (M = Al, Ga, and In) were similar, as the ideality of the square pyramidal structures is indicated by small τ values (near 0) [19].The displacements of M from the best N2S2 planes showed aluminum to be the most out of plane, ∆ = 0.616 Å, while Ga and In showed displacements in the range of 0.53-0.55Å.These displacements are likewise seen in some pentacoordinate transition metal complexes, such as MN2S2, where M = [Co(NO)] 2+ (0.31 Å), [Fe(NO)] 2+ (0.55 Å), and [V≡O] 2+ (0.652 Å) [20,21].In all cases of A, B, and C, the two-carbon linker within the diazacycloheptane ring is on the same side as the M III -X bond vector.This means that the MN2C3 cyclohexane-type ring in the chair configuration is oriented "underneath" the N2S2 base of the square pyramidal X-MN2S2 structure.We saw no evidence of fluxionality in the solution.
Different from Ga and In, which are largely located on the midpoint line in the center of the N2S2 unit, Al showed a slight dissymmetry in its location, shifted towards the nitrogen atoms (Al-N is around 2.1 Å) and away from the sulfur atoms (Al-S is around 2.3 Å).This suggests a stronger electrostatic interaction to N from the Al III .Table S5 lists the comparative ratios of the M-N/M-S distances that emphasize the similarity of Ga and In and their greater affinity for sulfur.From Al, Ga, to In, the angles of ∠N-M-N decreased with a concomitant increase in ∠S-M-S.

Reactions of XMN2S2 with Ni(II) Sources
The optimal sized N2S2 cavity of the bme-dach and analogous ligands, as well as the electronic structure preference of d 8 Ni II , leads to a well-known library of square planar NiN2S2 complexes [7].Thus, we probed the possibility of Ni II replacement of the Group III metal ions in the XMN2S2 complexes.Such metal exchange studies were not easily performed in the transition metal complexes, as aggregation at the sulfur elements was prominent.
The addition of NiCl2 or Ni(BF4)2 to XM III N2S2 complexes resulted in the formation of NiN2S2 with M III replaced by Ni II , concomitant with a color change from colorless to dark maroon, illustrated in Figure 5.The resulting maroon product gave a yield of 75%-80% and was confirmed as the known NiN2S2 complex by mass spectroscopy (Figure S12) and X-ray structure analyses.To determine whether the XM III unit might remain in the N2S2 cavity with the attachment of Ni(diphos) to the nucleophilic sulfurs of XM III N2S2, we added the (diphos)NiCl2 complex to a solution of XM III N2S2.The product of that reaction was determined to be the well-known dinickel complex derived from two (diphos)NiCl2 complexes with Ni II moving into the tight binding site, replacing XM III , and the second nickel holding on to the diphos ligand, Figure 5.The stability of the NiN2S2 complex relative to XM III N2S2 was confirmed.The geometries of XMN 2 S 2 (M = Al, Ga, and In) were similar, as the ideality of the square pyramidal structures is indicated by small τ values (near 0) [19].The displacements of M from the best N 2 S 2 planes showed aluminum to be the most out of plane, ∆ = 0.616 Å, while Ga and In showed displacements in the range of 0.53-0.55Å.These displacements are likewise seen in some pentacoordinate transition metal complexes, such as MN 2 S 2 , where M = [Co(NO)] 2+ (0.31 Å), [Fe(NO)] 2+ (0.55 Å), and [V≡O] 2+ (0.652 Å) [20,21].In all cases of A, B, and C, the two-carbon linker within the diazacycloheptane ring is on the same side as the M III -X bond vector.This means that the MN 2 C 3 cyclohexane-type ring in the chair configuration is oriented "underneath" the N 2 S 2 base of the square pyramidal X-MN 2 S 2 structure.We saw no evidence of fluxionality in the solution.
Different from Ga and In, which are largely located on the midpoint line in the center of the N 2 S 2 unit, Al showed a slight dissymmetry in its location, shifted towards the nitrogen atoms (Al-N is around 2.1 Å) and away from the sulfur atoms (Al-S is around 2.3 Å).This suggests a stronger electrostatic interaction to N from the Al III .Table S5 lists the comparative ratios of the M-N/M-S distances that emphasize the similarity of Ga and In and their greater affinity for sulfur.From Al, Ga, to In, the angles of ∠N-M-N decreased with a concomitant increase in ∠S-M-S.

Reactions of XMN 2 S 2 with Ni(II) Sources
The optimal sized N 2 S 2 cavity of the bme-dach and analogous ligands, as well as the electronic structure preference of d 8 Ni II , leads to a well-known library of square planar NiN 2 S 2 complexes [7].Thus, we probed the possibility of Ni II replacement of the Group III metal ions in the XMN 2 S 2 complexes.Such metal exchange studies were not easily performed in the transition metal complexes, as aggregation at the sulfur elements was prominent.
The addition of NiCl 2 or Ni(BF 4 ) 2 to XM III N 2 S 2 complexes resulted in the formation of NiN 2 S 2 with M III replaced by Ni II , concomitant with a color change from colorless to dark maroon, illustrated in Figure 5.The resulting maroon product gave a yield of 75-80% and was confirmed as the known NiN 2 S 2 complex by mass spectroscopy (Figure S12) and X-ray structure analyses.To determine whether the XM III unit might remain in the N 2 S 2 cavity with the attachment of Ni(diphos) to the nucleophilic sulfurs of XM III N 2 S 2 , we added the (diphos)NiCl 2 complex to a solution of XM III N 2 S 2 .The product of that reaction was determined to be the well-known dinickel complex derived from two (diphos)NiCl 2 complexes with Ni II moving into the tight binding site, replacing XM III , and the second nickel holding on to the diphos ligand, Figure 5.The stability of the NiN 2 S 2 complex relative to XM III N 2 S 2 was confirmed.

Nucleophilicity of S Lone Pairs on XMN2S2 Towards the Soft Receiver W(CO)5
The stability and electronic reporting capability of tungsten carbonyls have proven useful to rank the donor ability of various MN2S2 metalloligands.The labile ligand synthons W(CO)5(solv) and W(CO)4(solv)2 were generated by appropriate methods [22,23].Carbon monoxide was lost in solutions of W(CO)6 in THF under UV light, after which the golden yellow solution showed the ν(CO) C4v pattern typical of W(CO)5(THF) and was transferred via cannula to a solution of XMN2S2 (XM = ClGa/ClIn).Over the course of several hours at room temperature, with no further photolysis, the color changed to brown-orange.As shown in the overlaid spectra in Figure 6, the shift in ν(CO) was not large, indicating the donor ability of THF and monodentate XMN2S2 toward W(CO)5 is similar and weak.In contrast, NiN2S2 reacted with W(CO)5(THF), producing a much greater change in ν(CO) of 2062 (w), 1922 (s), and 1884 (m) [23].The MeAlN2S2 metalloligand itself is not stable under UV light and gave products of degradation in the presence of W(CO)5(THF).The stability and electronic reporting capability of tungsten carbonyls have proven useful to rank the donor ability of various MN 2 S 2 metalloligands.The labile ligand synthons W(CO) 5 (solv) and W(CO) 4 (solv) 2 were generated by appropriate methods [22,23].Carbon monoxide was lost in solutions of W(CO) 6 in THF under UV light, after which the golden yellow solution showed the ν(CO) C 4v pattern typical of W(CO) 5 (THF) and was transferred via cannula to a solution of XMN 2 S 2 (XM = ClGa/ClIn).Over the course of several hours at room temperature, with no further photolysis, the color changed to brown-orange.As shown in the overlaid spectra in Figure 6, the shift in ν(CO) was not large, indicating the donor ability of THF and monodentate XMN 2 S 2 toward W(CO) 5 is similar and weak.In contrast, NiN 2 S 2 reacted with W(CO) 5 (THF), producing a much greater change in ν(CO) of 2062 (w), 1922 (s), and 1884 (m) [23].The MeAlN 2 S 2 metalloligand itself is not stable under UV light and gave products of degradation in the presence of W(CO) 5 (THF).

Nucleophilicity of S Lone Pairs on XMN2S2 Towards the Soft Receiver W(CO)5
The stability and electronic reporting capability of tungsten carbonyls have proven useful to rank the donor ability of various MN2S2 metalloligands.The labile ligand synthons W(CO)5(solv) and W(CO)4(solv)2 were generated by appropriate methods [22,23].Carbon monoxide was lost in solutions of W(CO)6 in THF under UV light, after which the golden yellow solution showed the ν(CO) C4v pattern typical of W(CO)5(THF) and was transferred via cannula to a solution of XMN2S2 (XM = ClGa/ClIn).Over the course of several hours at room temperature, with no further photolysis, the color changed to brown-orange.As shown in the overlaid spectra in Figure 6, the shift in ν(CO) was not large, indicating the donor ability of THF and monodentate XMN2S2 toward W(CO)5 is similar and weak.In contrast, NiN2S2 reacted with W(CO)5(THF), producing a much greater change in ν(CO) of 2062 (w), 1922 (s), and 1884 (m) [23].The MeAlN2S2 metalloligand itself is not stable under UV light and gave products of degradation in the presence of W(CO)5(THF).Orange sheet crystals of ClGaN 2 S 2 •W(CO) 5 were obtained from THF solution by hexane layering.The X-ray diffraction study showed that the geometry and metric parameters were largely the same in the free and S-bound monodentate complexes.The Ga-S 1 bridge bond was elongated compared with the ClGaN 2 S 2 structure.The N 2 S 2 "plane" was quite distorted, giving the ClGaN 2 S 2 a geometry between square pyramidal and trigonal bipyramidal with the τ value 0.50, as shown in Figure S18 [19].Gallium(III) was significantly more displaced from the N 2 S 2 "plane", giving a deviation of over 0.1 Å (0.529 Å in ClGaN 2 S 2 to 0.654 Å in ClGaN 2 S 2 •W(CO) 5 ).The distance between Ga and W was 4.309 Å.
On further exposing ClGaN 2 S 2 •W(CO) 5 to UV light, a second CO was lost, yielding ClGaN 2 S 2 •W(CO) 4 .The ν(CO) IR spectrum displays a four-band pattern typical of C 2v metal carbonyl derivatives; the values are 2004 (w), 1929 (m), 1890 (m), and 1846 (m) cm −1 with the color changing to light orange, Figure 7. Similar to the ClGaN 2 S 2 •W(CO) 5 complex, the ClInN 2 S 2 •W(CO) 5 converted to ClInN 2 S 2 •W(CO) 4 under light, giving ν (CO) values of 2007 (w), 1929 (m), 1894 (m), and 1850 (m) cm −1 , as shown in Figure S17.The final product was confirmed by − ESI-Mass, and details are given in the Supplementary Materials.The experimental results indicate that the thiolate sulfurs in the XMN 2 S 2 complexes are still sufficiently active to bind another metal.However, the binding ability is weak.The latter statement was confirmed by the reversibility of the CO loss.On bubbling CO through the solution of ClGaN 2 S 2 •W(CO) 4 , the pentacarbonyl was readily reclaimed.
Orange sheet crystals of ClGaN2S2•W(CO)5 were obtained from THF solution by hexane layering.The X-ray diffraction study showed that the geometry and metric parameters were largely the same in the free and S-bound monodentate complexes.The Ga-S1 bridge bond was elongated compared with the ClGaN2S2 structure.The N2S2 "plane" was quite distorted, giving the ClGaN2S2 a geometry between square pyramidal and trigonal bipyramidal with the τ value 0.50, as shown in Figure S18 [19].Gallium(III) was significantly more displaced from the N2S2 "plane", giving a deviation of over 0.1 Å (0.529 Å in ClGaN2S2 to 0.654 Å in ClGaN2S2•W(CO)5).The distance between Ga and W was 4.309 Å.
On further exposing ClGaN2S2•W(CO)5 to UV light, a second CO was lost, yielding ClGaN2S2 W(CO)4.The ν(CO) IR spectrum displays a four-band pattern typical of C2v metal carbonyl derivatives; the values are 2004 (w), 1929 (m), 1890 (m), and 1846 (m) cm −1 with the color changing to light orange, Figure 7. Similar to the ClGaN2S2•W(CO)5 complex, the ClInN2S2•W(CO)5 converted to ClInN2S2•W(CO)4 under light, giving ν (CO) values of 2007 (w), 1929 (m), 1894 (m), and 1850 (m) cm −1 , as shown in Figure S17.The final product was confirmed by − ESI-Mass, and details are given in the Supplementary Materials.The experimental results indicate that the thiolate sulfurs in the XMN2S2 complexes are still sufficiently active to bind another metal.However, the binding ability is weak.The latter statement was confirmed by the reversibility of the CO loss.On bubbling CO through the solution of ClGaN2S2•W(CO)4, the pentacarbonyl was readily reclaimed.

Materials and Methods
All reagents and solvents were obtained from commercial sources.All solvents were purified and dried by an MBRAUN Manual Solvent Purification System (MBRAUN, NH, USA) packed with Alcoa F200 activated alumina desiccant.All reactions and operations were carried out on a double manifold Schlenk vacuum line or in a glovebox under a N2 or Ar atmosphere.
Solution infrared spectra were recorded on a Bruker Tensor 37 Fourier transform IR (FTIR) spectrometer (Billerica, MA, USA) using a CaF2 cell with a 0.

Materials and Methods
All reagents and solvents were obtained from commercial sources.All solvents were purified and dried by an MBRAUN Manual Solvent Purification System (MBRAUN, NH, USA) packed with Alcoa F200 activated alumina desiccant.All reactions and operations were carried out on a double manifold Schlenk vacuum line or in a glovebox under a N 2 or Ar atmosphere.
Solution infrared spectra were recorded on a Bruker Tensor 37 Fourier transform IR (FTIR) spectrometer (Billerica, MA, USA) using a CaF 2 cell with a 0.2 mm path length.Both High Resolution and Low Resolution Mass spectrometry (Thermo Fisher Q Exactive Mass Spectrometer, ESI-MS, (IET, IL, USA) were performed in the Laboratory for Biological Mass Spectrometry at Texas A&M University.Data collections for X-ray structure-determination were carried out using Bruker APEX2 (Billerica, MA, USA) or Venture with a graphite monochromated radiation source (λ = 0.71073 Å).All crystals were coated in paraffin oil and mounted on a nylon loop and placed under streaming N 2 (110/150K).The structures were solved by direct methods (SHELXS-97) and refined by standard Fourier techniques against F square with a full-matrix least-squares algorithm using SHELXL-97 and the WinGX (1.80.05) software package (University Of Glasgow, Scotland, UK).Hydrogen atoms were placed in calculated positions and refined with a riding model.Graphical representations were prepared with ORTEP-III.Crystallographic data (including structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos.1943010-1943013.

Reactions of XMN 2 S 2 with Ni(II)
The white solid of XMN 2 S 2 (0.1 mmol) was placed in a 50 mL flask, and 10 mL of a MeOH/MeCN (1:1) mixture was added.The green solution of NiCl 2 •6H 2 O (0.1 mmol, 24 mg) in MeOH was then transferred to the 50 mL flask.The color immediately changed to maroon, which is typical of NiN 2 S 2 , and this product was confirmed by its X-ray structure.The same product, NiN 2 S 2 , was also formed by adding Ni(BF 4 ) 2 using the same method.
Similar to the reaction with the chloride salt of Ni II , the white solid of XMN 2 S 2 (0.1 mmol) was stirred with 10 mL MeCN in the 50 mL round flask.The NiP 2 Cl 2 (P 2 = diphos or 1,1 -diphenylphosphinoethane, 0.105 g, 0.2 mmol) was dissolved in 10 mL MeCN and transferred to the flask.The resulting dark brown solution was dried in vacuo, giving a dark brown solid.Dark brown needle crystals were formed under Et 2 O diffusion into a concentrated CH 3 CN solution, proven to be the dinickel complex by X-ray structure and + ESI-MS analyses.

Synthesis of ClMW(CO) 5
The W(CO) 5 (solv) was generated in situ by W(CO) 6 in THF under UV light, after which the golden yellow solution was directly transferred to a white suspension of XMN 2 S 2 (M = GaCl/InCl) in THF.Then, the mixture was stirred at room temperature and monitored by FTIR.After several hours, the color changed to a clear brown-orange and without further ν(CO) change.IR (cm −1 ): ClGaN 2 S 2 W(CO) 5 ,

Figure 2 .
Figure 2. Examples of bi-and trimetallic complexes accessed by connecting the MN2S2 metallodithiolate ligand to various transition metal receivers, adapted from [7].

Figure 1 .
Figure 1.Structure and function of the dinickel enzyme active site of acetyl co-A synthase [3].

Figure 1 .
Figure 1.Structure and function of the dinickel enzyme active site of acetyl co-A synthase [3].

Figure 2 .
Figure 2. Examples of bi-and trimetallic complexes accessed by connecting the MN2S2 metallodithiolate ligand to various transition metal receivers, adapted from [7].

Figure 2 .
Figure 2. Examples of bi-and trimetallic complexes accessed by connecting the MN 2 S 2 metallodithiolate ligand to various transition metal receivers, adapted from [7].

Figure 4 .
Figure 4. Molecular structures of XMN2S2 complexes from XRD analysis with selected metric data.

Figure 4 .
Figure 4. Molecular structures of XMN 2 S 2 complexes from XRD analysis with selected metric data.