Metal Coordination Enhances Chalcogen Bonds: CSD Survey and Theoretical Calculations

In this study the ability of metal coordinated Chalcogen (Ch) atoms to undergo Chalcogen bonding (ChB) interactions has been evaluated at the PBE0-D3/def2-TZVP level of theory. An initial CSD (Cambridge Structural Database) inspection revealed the presence of square planar Pd/Pt coordination complexes where divalent Ch atoms (Se/Te) were used as ligands. Interestingly, the coordination to the metal center enhanced the σ-hole donor ability of the Ch atom, which participates in ChBs with neighboring units present in the X-ray crystal structure, therefore dictating the solid state architecture. The X-ray analyses were complemented with a computational study (PBE0-D3/def2-TZVP level of theory), which shed light into the strength and directionality of the ChBs studied herein. Owing to the new possibilities that metal coordination offers to enhance or modulate the σ-hole donor ability of Chs, we believe that the findings presented herein are of remarkable importance for supramolecular chemists as well as for those scientists working in the field of solid state chemistry.

Nowadays a remarkable progress has been achieved regarding the supramolecular chemistry of chalcogen bonding, particularly in regulating and fine tuning novel chemical R groups attached to the Ch atom: The substituents (R) effect to the Ch···A bond features have been analysed from a theoretical perspective [58][59][60][61][62], evidencing that EWG enhance the strength of the ChB through the formation of larger and deeper σ-hole(s). Conversely, the use of electron-donating groups (EDG) resulted in a weakening of the interaction. Both types of substituents have been used to correlate structure-property functions, as well as a source of stabilization of the secondary coordination sphere in metal complexes. Figure 1 shows several Molecular Electrostatic Potential (MEP) surfaces of the -CH 3 and -CF 3 substituted Selenium and Tellurium derivatives. As noted, two σ-holes are located on each Ch atom on the extension of the C-Se and C-Te bonds and their MEP values become more positive ongoing from Se to Te, in agreement with the atomic polarizabilities discussed above. Moreover, the use of strong EWG (e.g., -CF 3 ) increases the potential of the σ-hole, thus enhancing the σ-hole donor ability of the Ch atom and strengthening the interaction. Finally, using EDG as substituents of the Lewis base can also enhance the ChB interaction by increasing the nucleophilicity of the electron rich moiety [63,64].  [68].
In this article, we illustrate several X-ray structures to highlight that ChBs in metal complexes can be used as an effective tool in crystal engineering and in the generation of functional materials. Particularly, we have focused our attention on a divalent Ch moiety coordinated to a metal center, which certainly increases the R-Ch σ-hole, leading to new opportunities to use ChBs as a supramolecular tool in coordination chemistry [78,79]. To illustrate this idea, two examples have been selected (see Figure 2), where a bidentate Se/Te ligand is coordinated to a Pd 2+ ion in addition to two chloride ions in a square planar geometry. Upon coordination, there is one Se/Te σ-hole still available to undergo ChB interactions, which exhibits an enhanced electrostatic potential (up to 14 kcal/mol) compared to the uncoordinated ligand. In this context, the Cambridge Structural Database (CSD) [80] has been inspected and a series of selected examples where Se or Te atoms coordinated to square planar metal coordinated behave as an electrophilic center (chalcogen bond donor) have been discussed. In a parallel way to X-ray analysis, the presence of intermolecular chalcogen bonding in metal organic complexes has been also analyzed from a theoretical perspective. The selection of square planar complexes instead of octahedral is to prevent steric effects that may hamper the formation of ChB interactions, as represented in Scheme 1. We believe
In this article, we illustrate several X-ray structures to highlight that ChBs in metal complexes can be used as an effective tool in crystal engineering and in the generation of functional materials. Particularly, we have focused our attention on a divalent Ch moiety coordinated to a metal center, which certainly increases the R-Ch σ-hole, leading to new opportunities to use ChBs as a supramolecular tool in coordination chemistry [77,78]. To illustrate this idea, two examples have been selected (see Figure 2), where a bidentate Se/Te ligand is coordinated to a Pd 2+ ion in addition to two chloride ions in a square planar geometry. Upon coordination, there is one Se/Te σ-hole still available to undergo ChB interactions, which exhibits an enhanced electrostatic potential (up to 14 kcal/mol) compared to the uncoordinated ligand.  [68].
In this article, we illustrate several X-ray structures to highlight that ChBs in metal complexes can be used as an effective tool in crystal engineering and in the generation of functional materials. Particularly, we have focused our attention on a divalent Ch moiety coordinated to a metal center, which certainly increases the R-Ch σ-hole, leading to new opportunities to use ChBs as a supramolecular tool in coordination chemistry [78,79]. To illustrate this idea, two examples have been selected (see Figure 2), where a bidentate Se/Te ligand is coordinated to a Pd 2+ ion in addition to two chloride ions in a square planar geometry. Upon coordination, there is one Se/Te σ-hole still available to undergo ChB interactions, which exhibits an enhanced electrostatic potential (up to 14 kcal/mol) compared to the uncoordinated ligand. In this context, the Cambridge Structural Database (CSD) [80] has been inspected and a series of selected examples where Se or Te atoms coordinated to square planar metal coordinated behave as an electrophilic center (chalcogen bond donor) have been discussed. In a parallel way to X-ray analysis, the presence of intermolecular chalcogen bonding in metal organic complexes has been also analyzed from a theoretical perspective. The selection of square planar complexes instead of octahedral is to prevent steric effects that may hamper the formation of ChB interactions, as represented in Scheme 1. We believe In this context, the Cambridge Structural Database (CSD) [79] has been inspected and a series of selected examples where Se or Te atoms coordinated to square planar metal coordinated behave as an electrophilic center (chalcogen bond donor) have been discussed. In a parallel way to X-ray analysis, the presence of intermolecular chalcogen bonding in metal organic complexes has been also analyzed from a theoretical perspective. The selection of square planar complexes instead of octahedral is to prevent steric effects that may hamper the formation of ChB interactions, as represented in Scheme 1. We believe this review article will attract the attention of both theoreticians and experimentalists to investigate the ChBs in coordination compounds, which can be used as a new synthon in crystal engineering and improve the functional properties of materials.
this review article will attract the attention of both theoreticians and experimentalists to investigate the ChBs in coordination compounds, which can be used as a new synthon in crystal engineering and improve the functional properties of materials. Scheme 1. Schematic representation of a ChB interaction in a Ch-coordinated octahedral (left) and square planar (right) complex.

Computational Methods
Calculations regarding the supramolecular complexes have been carried out at the PBE0 [81,82]-D3 [83]/def2-TZVP [67] level of theory using the program Turbomole 7.2 (Karlsruhe, Germany) [68]. The binding energy values (ΔEBSSE) were calculated as the energy difference between the optimized structures of the complex and isolated monomers Finally, the Bader's "Atoms in molecules" theory [86,87] has been used to analyze and describe the interactions discussed in this work using the AIMall calculation package [88]. The PBE0/def2-TZVP level of theory was used for the wavefunction analysis as well as for the NBO charge analysis (also using Gaussian-16 software).

CSD Search
We have inspected the CSD and found 103 structures where either divalent Se or Te is coordinated to square planar metal centers, which were in all cases Pt or Pd. No examples with other typical square planar metals like Ni and Rh were found. Remarkably, in 73 out of 103 structures, we detected the existence of ChB interactions, using the following geometric criteria: Ch···X distance shorter than the sum of van der Waals radii plus 0.2 Å (ΣRvdw + 0.2) and ∡ R-Ch···X greater than 160 degrees (X = O, C, Pt, Pd, Cl and I). Table S1 compiles the CSD reference codes, geometric features and donor-acceptor atoms. Most of the hits are observed for Se (40 structures) and chloride as electron donor (52 structures). Other electron donors are Br, I, O and C-atoms belonging to electron rich aromatic rings. Interestingly, we have also observed some structures where it is the metal center that acts as electron donor (3 for Pd and 3 for Pt). The geometric features, energies and characterization of the ChBs are provided in the following sections.

Computational Methods
Calculations regarding the supramolecular complexes have been carried out at the PBE0 [80,81] To compute the contribution of the Chalcogen bond contacts (in kcal/mol) the following equations were used [84]: Finally, the Bader's "Atoms in molecules" theory [85,86] has been used to analyze and describe the interactions discussed in this work using the AIMall calculation package [87]. The PBE0/def2-TZVP level of theory was used for the wavefunction analysis as well as for the NBO charge analysis (also using Gaussian-16 software).

CSD Search
We have inspected the CSD and found 103 structures where either divalent Se or Te is coordinated to square planar metal centers, which were in all cases Pt or Pd. No examples with other typical square planar metals like Ni and Rh were found. Remarkably, in 73 out of 103 structures, we detected the existence of ChB interactions, using the following geometric criteria: Ch···X distance shorter than the sum of van der Waals radii plus 0.2 Å (ΣR vdw + 0.2) and R-Ch···X greater than 160 degrees (X = O, C, Pt, Pd, Cl and I). Table S1 compiles the CSD reference codes, geometric features and donor-acceptor atoms. Most of the hits are observed for Se (40 structures) and chloride as electron donor (52 structures). Other electron donors are Br, I, O and C-atoms belonging to electron rich aromatic rings. Interestingly, we have also observed some structures where it is the metal center that acts as electron donor (3 for Pd and 3 for Pt). The geometric features, energies and characterization of the ChBs are provided in the following sections. opposite to the C-Se bond, thus forming a ChB. The MEP surface (Figure 3b) of this compound reveals the existence of three σ-holes opposite to the three covalent bonds, including the coordination Pt-Se bond. It can be observed that the intensity of the σ-hole opposite to the C-Se bond is large, which is quite unexpected considering the electronegativity of carbon (C-Se bond is not much polarized). This is explained by the coordination of Se to the Pt that increases the Lewis acidity of the Se-atom. It can be also observed that the approximation of the O-atom to the Se could not be possible for a hypothetical octahedral complex due to the presence of an apical ligand. Figure 3a shows a self-assembled dimer that is formed in the solid state of dichloro-(2,3:7,8-bis(methylenedioxy)-selenanthrene-Se,Se′)-platinum(II) (refcode DUWMEN [89]). It can be observed that one O-atom of the methylenedioxy group is located opposite to the C-Se bond, thus forming a ChB. The MEP surface (Figure 3b) of this compound reveals the existence of three σ-holes opposite to the three covalent bonds, including the coordination Pt-Se bond. It can be observed that the intensity of the σ-hole opposite to the C-Se bond is large, which is quite unexpected considering the electronegativity of carbon (C-Se bond is not much polarized). This is explained by the coordination of Se to the Pt that increases the Lewis acidity of the Se-atom. It can be also observed that the approximation of the O-atom to the Se could not be possible for a hypothetical octahedral complex due to the presence of an apical ligand.  [90] and Pt for XUFHIQ [90]). Both structures form very similar self-assembled dimers in the solid state where two symmetrically equivalent Se···O contacts are established. Both dimers also present metal···metal (M···M) interactions with distances shorter than 3.45 Å . The Se···O distances are shorter in XUF-HEM ( Figure 4a) structure, likely influenced by the shorter Pd···Pd distance. The directionality of the ChBs is worse (with respect to linearity) than the observed in DUWMEN structure (Figure 4b), which is also due to the restriction imposed by the M···M interaction.  [89] and Pt for XUFHIQ [89]). Both structures form very similar self-assembled dimers in the solid state where two symmetrically equivalent Se···O contacts are established. Both dimers also present metal···metal (M···M) interactions with distances shorter than 3.45 Å. The Se···O distances are shorter in XUFHEM ( Figure 4a) structure, likely influenced by the shorter Pd···Pd distance. The directionality of the ChBs is worse (with respect to linearity) than the observed in DUWMEN structure (Figure 4b), which is also due to the restriction imposed by the M···M interaction. Figure 3a shows a self-assembled dimer that is formed in the solid state of dichloro-(2,3:7,8-bis(methylenedioxy)-selenanthrene-Se,Se′)-platinum(II) (refcode DUWMEN [89]). It can be observed that one O-atom of the methylenedioxy group is located opposite to the C-Se bond, thus forming a ChB. The MEP surface (Figure 3b) of this compound reveals the existence of three σ-holes opposite to the three covalent bonds, including the coordination Pt-Se bond. It can be observed that the intensity of the σ-hole opposite to the C-Se bond is large, which is quite unexpected considering the electronegativity of carbon (C-Se bond is not much polarized). This is explained by the coordination of Se to the Pt that increases the Lewis acidity of the Se-atom. It can be also observed that the approximation of the O-atom to the Se could not be possible for a hypothetical octahedral complex due to the presence of an apical ligand.  [90] and Pt for XUFHIQ [90]). Both structures form very similar self-assembled dimers in the solid state where two symmetrically equivalent Se···O contacts are established. Both dimers also present metal···metal (M···M) interactions with distances shorter than 3.45 Å . The Se···O distances are shorter in XUF-HEM ( Figure 4a) structure, likely influenced by the shorter Pd···Pd distance. The directionality of the ChBs is worse (with respect to linearity) than the observed in DUWMEN structure (Figure 4b), which is also due to the restriction imposed by the M···M interaction.

Halogen as Electron Donor
As commented above, most hits from the CSD search present an halogen atom as electron donor, basically because chloride is widely used in the synthesis of Pd and Pt complexes. Figure 5a highlights a self-assembled dimer that is formed in the solid state of (1,2-bis(phenylseleno)benzene)-dichlorido-palladium(II) (refcode XILFEE [90]), selected as representative X-ray structure with Se···Cl contacts. It can be observed that the chloride ligands are located opposite to the C-Se bonds, thus forming four concurrent ChBs and one Pd···Pd contact. The MEP surface of this compound (see Figure 5b) reveals the existence of two σ-holes opposite to (i) one C-Se covalent bond and (ii) to one Pt-Se coordination bond. The second C-Se σ-hole is buried by the large negative belt of Cl. It can be observed that the MEP value at the σ-hole opposite to the Pd-Se bond is larger likely due to the contribution of the nearby C-H bond.

Halogen as Electron Donor
As commented above, most hits from the CSD search present an halogen atom as electron donor, basically because chloride is widely used in the synthesis of Pd and Pt complexes. Figure 5a highlights a self-assembled dimer that is formed in the solid state of (1,2-bis(phenylseleno)benzene)-dichlorido-palladium(II) (refcode XILFEE [91]), selected as representative X-ray structure with Se···Cl contacts. It can be observed that the chloride ligands are located opposite to the C-Se bonds, thus forming four concurrent ChBs and one Pd···Pd contact. The MEP surface of this compound (see Figure 5b) reveals the existence of two σ-holes opposite to (i) one C-Se covalent bond and (ii) to one Pt-Se coordination bond. The second C-Se σ-hole is buried by the large negative belt of Cl. It can be observed that the MEP value at the σ-hole opposite to the Pd-Se bond is larger likely due to the contribution of the nearby C-H bond.  Figure 6, in all cases the adducts are self-assembled dimers. In the dimer of dichloro-(diphenyl(2-(phenylselanyl)phenyl)phosphine)-platinum (PUYWUC [92], Figure 6a), the Pt···Pt interaction is not established and the formation of the dimer is dominated by ChBs. In the other two structures (refcodes TAFWIJ [93], Figure 6b and EPULIM [94], Figure 6c), Pd···Pd interactions coexist with the Se···Cl contacts. Among them, TAFWIJ structure is the one exhibiting the highest directionality (∡C-Se···Cl = 176.1°) and the shortest M···M distance.

Halogen as Electron Donor
As commented above, most hits from the CSD search present an halogen atom as electron donor, basically because chloride is widely used in the synthesis of Pd and Pt complexes. Figure 5a highlights a self-assembled dimer that is formed in the solid state of (1,2-bis(phenylseleno)benzene)-dichlorido-palladium(II) (refcode XILFEE [91]), selected as representative X-ray structure with Se···Cl contacts. It can be observed that the chloride ligands are located opposite to the C-Se bonds, thus forming four concurrent ChBs and one Pd···Pd contact. The MEP surface of this compound (see Figure 5b) reveals the existence of two σ-holes opposite to (i) one C-Se covalent bond and (ii) to one Pt-Se coordination bond. The second C-Se σ-hole is buried by the large negative belt of Cl. It can be observed that the MEP value at the σ-hole opposite to the Pd-Se bond is larger likely due to the contribution of the nearby C-H bond.  Figure 6, in all cases the adducts are self-assembled dimers. In the dimer of dichloro-(diphenyl(2-(phenylselanyl)phenyl)phosphine)-platinum (PUYWUC [92], Figure 6a), the Pt···Pt interaction is not established and the formation of the dimer is dominated by ChBs. In the other two structures (refcodes TAFWIJ [93], Figure 6b and EPULIM [94], Figure 6c), Pd···Pd interactions coexist with the Se···Cl contacts. Among them, TAFWIJ structure is the one exhibiting the highest directionality (∡C-Se···Cl = 176.1°) and the shortest M···M distance.  Figure 7a shows a dimer extracted from an 1D supramolecular assembly that propagates in the crystal packing of chloro- [8-({[2-(phenylselanyl)ethyl]imino}methyl)naphthalen-1-yl]-palladium(II) complex (refcode BEJWIZ [94]). The C-atom bonded to the Pd is located opposite to the C-Se bond, thus playing the electron donor role in this ChB interaction, which is facilitated by the anionic nature of this C-atom. The MEP surface (Figure 7b) shows a small σ-hole opposite to the C-Se bond that merges with the large blue region under the influence of the methylene group.  Figure 7a shows a dimer extracted from an 1D supramolecular assembly that propagates in the crystal packing of chloro- [8-({[2-(phenylselanyl)ethyl]imino}methyl)naphthalen-1-yl]-palladium(II) complex (refcode BEJWIZ [95]). The C-atom bonded to the Pd is located opposite to the C-Se bond, thus playing the electron donor role in this ChB interaction, which is facilitated by the anionic nature of this C-atom. The MEP surface ( Figure  7b) shows a small σ-hole opposite to the C-Se bond that merges with the large blue region under the influence of the methylene group. Other two examples of X-ray structures exhibiting Se···C contacts, where the electron donor C-atom belongs to an aromatic system, are given in Figure 8. It is interesting to highlight that for chloro-(2,4-di-t-butyl-6-(((2-(phenylselanyl)ethyl)imino)methyl)phenolato)-palladium(II) structure (Figure 8a, refcode COKFIT [96]), the self-assembled dimer does not present M···M interactions and the C-atom is located opposite to the Pd-Se bond. Therefore, in this compound, the third σ-hole at the Se-atom that emerges upon Pdcomplexation is responsible for the formation of the dimers. In this example, the ChBs are the only forces governing the assembly.  Other two examples of X-ray structures exhibiting Se···C contacts, where the electron donor C-atom belongs to an aromatic system, are given in Figure 8. It is interesting to highlight that for chloro-(2,4-di-t-butyl-6-(((2-(phenylselanyl)ethyl)imino)methyl)phenolato)palladium(II) structure (Figure 8a, refcode COKFIT [95]), the self-assembled dimer does not present M···M interactions and the C-atom is located opposite to the Pd-Se bond. Therefore, in this compound, the third σ-hole at the Se-atom that emerges upon Pd-complexation is responsible for the formation of the dimers. In this example, the ChBs are the only forces governing the assembly.  [95]). The C-atom bonded to the Pd is located opposite to the C-Se bond, thus playing the electron donor role in this ChB interaction, which is facilitated by the anionic nature of this C-atom. The MEP surface ( Figure  7b) shows a small σ-hole opposite to the C-Se bond that merges with the large blue region under the influence of the methylene group. Other two examples of X-ray structures exhibiting Se···C contacts, where the electron donor C-atom belongs to an aromatic system, are given in Figure 8. It is interesting to highlight that for chloro-(2,4-di-t-butyl-6-(((2-(phenylselanyl)ethyl)imino)methyl)phenolato)-palladium(II) structure (Figure 8a, refcode COKFIT [96]), the self-assembled dimer does not present M···M interactions and the C-atom is located opposite to the Pd-Se bond. Therefore, in this compound, the third σ-hole at the Se-atom that emerges upon Pdcomplexation is responsible for the formation of the dimers. In this example, the ChBs are the only forces governing the assembly. For the other example (TOQPEV [97], Figure 8b), in addition to the ChBs opposite to the C-Se bonds, an M···M interaction is established further supporting the formation of the self-assembled dimer. Curiously, the Pd-Se···C ChBs (dimer of COKFIT) are more directional than the C-Se···C (dimer of TOPQEV).

Metal as Electron Donor
The role of metal centers in d 8 or d 10 configuration as electron donors has been recently analyzed for halogen bonding interactions [97]. However, similar investigations for chalcogen bonding are unprecedented. In this section, we show several examples where Pt or Pd act as electron donors in ChBs. Figure 9a shows a self-assembled dimer observed in the solid state of refcode VUGWOK [98]. The Pd is located opposite to the C-Se bond, thus acting as electron donor in the ChB interaction. The MEP surface (Figure 9b) shows a small σ-hole opposite to the Pd-Se bond (+8.1 kcal/mol) and a more intense one opposite to a C-Se bond (+16.9 kcal/mol).

Metal as Electron Donor
The role of metal centers in d 8 or d 10 configuration as electron donors has been recently analyzed for halogen bonding interactions [98]. However, similar investigations for chalcogen bonding are unprecedented. In this section, we show several examples where Pt or Pd act as electron donors in ChBs. Figure 9a shows a self-assembled dimer observed in the solid state of refcode VUGWOK [99]. The Pd is located opposite to the C-Se bond, thus acting as electron donor in the ChB interaction. The MEP surface (Figure 9b) shows a small σ-hole opposite to the Pd-Se bond (+8.1 kcal/mol) and a more intense one opposite to a C-Se bond (+16.9 kcal/mol).  Table 2, the list of CSD codes including compound names and formulas of the Se involving dimers is given.   Figure 10. It is worthy to comment that the C-Se···M angle is in all cases higher than 170 • , thus confirming the strong directionality of the interaction and that the nucleophilic metal center (filled d z 2 orbital) is pointing to the σ-hole opposite to the C-Se bond. For the three selected examples (QETFED [99] (Figure 10a), QETFAZ [99] (Figure 10b) and MELJUJ [100] (Figure 10c)) the distances are similar (around 3.7 Å). Finally, in Table 2, the list of CSD codes including compound names and formulas of the Se involving dimers is given.

Metal as Electron Donor
The role of metal centers in d 8 or d 10 configuration as electron donors has been recently analyzed for halogen bonding interactions [98]. However, similar investigations for chalcogen bonding are unprecedented. In this section, we show several examples where Pt or Pd act as electron donors in ChBs. Figure 9a shows a self-assembled dimer observed in the solid state of refcode VUGWOK [99]. The Pd is located opposite to the C-Se bond, thus acting as electron donor in the ChB interaction. The MEP surface (Figure 9b) shows a small σ-hole opposite to the Pd-Se bond (+8.1 kcal/mol) and a more intense one opposite to a C-Se bond (+16.9 kcal/mol).  (Figure 10c)) the distances are similar (around 3.7 Å ). Finally, in Table 2, the list of CSD codes including compound names and formulas of the Se involving dimers is given.

Tellurium Derivatives
In case of tellurium complexes, the CSD search shows that there are no examples involving oxygen as electron donor. In most of the hits, the electron donor is a halogen atom (Cl, Br or I), a π-system or a metal center (Pd and Pt) as discussed in the following sections. Figure 11a highlights a self-assembled dimer that is formed in the X-ray packing of bis(benzenetellurenyl iodide)-di-iodo-palladium(II) (refcode CUHMOJ [101]), selected as representative structure showing Te···I contacts. It can be observed that the iodide ligands are located approximately opposite to the C-Te bonds ( C-Te···I = 161.1 • ), thus forming four concurrent ChBs and one Pd···Pd contact (3.402 Å). The MEP surface of this compound (see Figure 11b) reveals the existence of two σ-holes opposite to (i) one C-Te covalent bond and (ii) to a Pd-Te coordination bond. The third σ-hole (expected opposite to the I-Te bond) is completely covered by the large and negative belt of I. It can be observed that the MEP value at the σ-hole opposite to the Pd-Te bond is smaller (+15.7 kcal/mol) than that opposite to the C-I bond (+18.8 kcal/mol).  [103]), the M···M interaction is not established and the dimer formation is basically dominated by Te···Cl ChBs that are less directional (157°) compared to those previously described for Se (Figure 12a). It is known that the directionality of ChBs decreases on going from S to Te because the size of the σ-hole is larger in the more polarizable Ch atoms. In the other two structures represented in

Carbon as Electron Donor
We have found only two structures in the database showing Te···C interactions, which are represented in Figure 13, refcodes SIDDAL [106] and WIXSEB [107]. In the former the chloro-(2-(1-((2-((4-methoxyphenyl)tellanyl)ethyl)amino)ethyl)phenolato)-palladium(II) forms discrete dimers with short Pd···Pd distance (3.203 Å ) reinforced by short and directional Te···Cl contacts (see Figure 13a) using the σ-hole opposite to the CAromatic-Te bond. In addition, these dimers self-assemble via the formation of ChBs using the σhole opposite the Caliphatic-Te bond where the electron donor C-atom belongs to the  [102]), the M···M interaction is not established and the dimer formation is basically dominated by Te···Cl ChBs that are less directional (157 • ) compared to those previously described for Se (Figure 12a). It is known that the directionality of ChBs decreases on going from S to Te because the size of the σ-hole is larger in the more polarizable Ch atoms. In the other two structures represented in Figure 12 (refcodes JAGZIA [103] (Figure 12b) and TAPYEO [104] (Figure 12c)), Pd···Pd and Pt···Pt interactions coexist with the Te···Br and Te···Cl contacts, respectively. JAGZIA structure is the one with higher directionality ( C-Te···Br = 171.7 • ) and shorter M···M distance (3.568 Å) whilst TAPYEO exhibits longer M···M distance (3.730 Å) and smaller angle ( C-Te···Cl = 154.7 • ).  [103]), the M···M interaction is not established and the dimer formation is basically dominated by Te···Cl ChBs that are less directional (157°) compared to those previously described for Se (Figure 12a). It is known that the directionality of ChBs decreases on going from S to Te because the size of the σ-hole is larger in the more polarizable Ch atoms. In the other two structures represented in

Carbon as Electron Donor
We have found only two structures in the database showing Te···C interactions, which are represented in Figure 13, refcodes SIDDAL [106] and WIXSEB [107]. In the former the chloro-(2-(1-((2-((4-methoxyphenyl)tellanyl)ethyl)amino)ethyl)phenolato)-palladium(II) forms discrete dimers with short Pd···Pd distance (3.203 Å ) reinforced by short and directional Te···Cl contacts (see Figure 13a) using the σ-hole opposite to the CAromatic-Te bond. In addition, these dimers self-assemble via the formation of ChBs using the σhole opposite the Caliphatic-Te bond where the electron donor C-atom belongs to the

Metal as Electron Donor
For tellurium, we have also found the interesting Ch···d 8 [M] ChB interaction commented above in two structures (M = Pt). Figure 14a shows a self-assembled dimer observed in the solid state of one of both (refcode ABOREP [107]). The Pt is located opposite to the C-Te bond, thus acting as electron donor in the ChB interaction. The MEP surface of chloro-(4-(2-(phenyltelluro)ethylimino)pentan-2-onato-N,O,Te)-platinum(II) complex (see Figure 14b) shows three σ-holes opposite to the three covalent bonds. The σ-hole opposite to the C aliphatic -Te bond presents the lowest value (+9.4 kcal/mol) while the other two exhibit similar and significantly larger MEP values (+21.9 and +25.1 kcal/mol) because they are under the influence of the CH bonds.

Metal as Electron Donor
For tellurium, we have also found the interesting Ch···d 8 [M] ChB interaction commented above in two structures (M = Pt). Figure 14a shows a self-assembled dimer observed in the solid state of one of both (refcode ABOREP [107]). The Pt is located opposite to the C-Te bond, thus acting as electron donor in the ChB interaction. The MEP surface of chloro-(4-(2-(phenyltelluro)ethylimino)pentan-2-onato-N,O,Te)-platinum(II) complex (see Figure 14b) shows three σ-holes opposite to the three covalent bonds. The σ-hole opposite to the Caliphatic-Te bond presents the lowest value (+9.4 kcal/mol) while the other two exhibit similar and significantly larger MEP values (+21.9 and +25.1 kcal/mol) because they are under the influence of the CH bonds. The other example of X-ray structure exhibiting Te···Pt ChBs is depicted in Figure 15, that corresponds to chloro-(N-(3-(phenyltellanyl)propyl)pyridine-2-carboxamidato)-platinum(II) complex (refcode CUHMAV [100]). It also forms self-assembled dimers in the solid state where the Pt atom is positioned opposite to the Ar-Te bond (3.715 Å ). The The other example of X-ray structure exhibiting Te···Pt ChBs is depicted in Figure 15, that corresponds to chloro-(N-(3-(phenyltellanyl)propyl)pyridine-2-carboxamidato)-platinum(II) complex (refcode CUHMAV [101]). It also forms self-assembled dimers in the solid state where the Pt atom is positioned opposite to the Ar-Te bond (3.715 Å). The directionality ( C-Te···Pt = 165.5 • ) of the ChB interactions suggests that the nucleophilic d z 2 orbital of Pt is pointing to the tellurium σ-hole, as it has been demonstrated for halogen bonded complexes [97]. Finally, in Table 3, the list of CSD codes including compound names and formulas of the Se involving dimers is given.  [98]. Finally, in Table 3, the list of CSD codes including compound names and formulas of the Se involving dimers is given.

Energetic Study
With the purpose to shed light on the nature and directionality of the ChBs studied herein we have performed a computational study on three selected dimers (XILFEE, MELJUJ and CUHMOJ). In Table 4 the binding energies (∆E BSSE ) and the contribution of the Chalcogen bond contacts (derived from the V(r) predictor) (∆E ChB ), equilibrium distances (d) as well as the value of the density at the bond critical point that characterizes the interaction (ρ·10 2 ) are included. Table 4. Binding energies and the contribution of the Chalcogen bond contacts (∆E BSSE and ∆E ChB in kcal/mol, respectively), equilibrium distances (d, in Å), C-Ch···X (X = Cl, O and I), angle (∠, in • ) and value of the density at the bond critical point (ρ 10 2 , in a.u.) for XILFEE, MELJUJ and CUHMOJ optimized dimers. The NBO charges of both chalcogen bond donor (Q D ) and acceptor (Q A ) atoms are also indicated in e. Firstly, the energetics, in all three calculated structures, large and attractive interaction energy values (ranging from −24.5 to −32.0 kcal/mol) were obtained, which involve the chalcogen bonding interactions as well as other noncovalent forces (e.g., hydrogen bonding, metal···metal interactions or π-π stacking). These results agree with very recent reports on similar supramolecular assemblies [108]. Secondly, the chalcogen bonding energies also achieved negative and attractive values (ranging from −2.7 to −7.9 kcal/mol). Finally, the NBO charges analyses resulted in positive values in the case of the ChB donor atom (Se/Te) of around 0.80e and negative ones in the case of the ChB acceptor atom (Cl, O and I), ranging between −0.2 and −0.6e. Figure 16a shows the QTAIM analysis of the XILFEE self-assembled dimer. The existence of the four Se···Cl ChBs is confirmed by the presence of four bond CPs and bond paths interconnecting the Se and Cl atoms. The QTAIM analysis also confirms the presence of a Pd···Pd interaction, characterized by a bond CP and bond path connecting both atoms. The dimerization energy is very large (−32.0 kcal/mol) due to the coexistence of five contacts, thus validating the structure directing role of such interactions in the solid state. Finally, Figure 16c shows the QTAIM analysis of the CUHMOJ self-assembled dimer. The existence of the four Te···I ChBs is corroborated by the presence of four bond CPs and bond paths interconnecting the Te and I atoms. The QTAIM analysis also evidences a Pd···Pd interaction, characterized by a bond CP and bond path connecting both atoms. The dimerization energy is large (−24.5 kcal/mol) due to the coexistence of five contacts, thus further supporting the structure directing role of such interactions in the solid state. The interaction energy is smaller (in absolute value) than that observed for the XILFEE On the other hand, in Figure 16b the QTAIM analysis of the MELJUJ structure is shown. As noticed, the presence of two Se···O ChBs is confirmed by the existence of two bond CPs and bond paths interconnecting the Se and O atoms. This result differs from the X-ray crystal structure, which exhibited a Se···Pd ChB. In addition, the existence of ancillary hydrogen bonding (HB) interactions between the CH groups from the aliphatic chain of the ligand and the chloride atoms was also confirmed by means of a bond CP and a bond path connecting both moieties. The dimerization energy is also large (−31.5 kcal/mol) due to the existence of both ChB and HB interactions, which contribute to the formation and stabilization of this supramolecular assembly.

AIM Analysis
Finally, Figure 16c shows the QTAIM analysis of the CUHMOJ self-assembled dimer. The existence of the four Te···I ChBs is corroborated by the presence of four bond CPs and bond paths interconnecting the Te and I atoms. The QTAIM analysis also evidences a Pd···Pd interaction, characterized by a bond CP and bond path connecting both atoms. The dimerization energy is large (−24.5 kcal/mol) due to the coexistence of five contacts, thus further supporting the structure directing role of such interactions in the solid state. The interaction energy is smaller (in absolute value) than that observed for the XILFEE dimer (see Figure 16a) described above, where a similar combination of interactions is observed. This is likely due to the fact that chloride is better electron donor than iodide (the MEP value at the negative belt is larger).

Concluding Remarks
In this review article, the CSD has been inspected to demonstrate the existence and strong directing role of ChBs interaction involving metal coordinated Se and Te atoms. The formation of the coordination bond has a double effect. First, it provides an additional σ-hole to the Se or Te atoms and, secondly, increases their Lewis base acidity, thus reinforcing their ability to establish ChBs. We have provided several examples of structures retrieved from the CSD that evidence the structure directing role of such interactions. Moreover, the energetic features of the ChBs have been computed and the interactions characterized by using the QTAIM. In some of these examples, the electron donor system is the metal center (Pd or Pt). This result is significant, since Ch···d 8 [M] interactions have not been studied before, as far as our knowledge extends. More investigation is thus needed in this direction, since it can be relevant in several fields like supramolecular chemistry, crystal engineering, coordination chemistry and supramolecular catalysis.