Coordination Behavior of [Cp″2Zr(µ1:1-As4)] towards Lewis Acids

The functionalization of the arsenic transfer reagent [Cp″2Zr(η1:1-As4)] (1) focuses on modifying its properties and enabling a broader scope of reactivity. The coordination behavior of 1 towards different Lewis-acidic transition metal complexes and main group compounds is investigated by experimental and computational studies. Depending on the steric requirements of the Lewis acids and the reaction temperature, a variety of new complexes with different coordination modes and coordination numbers could be synthesized. Depending on the Lewis acid (LA) used, a mono-substitution in [Cp″2Zr(µ,η1:1:1:1-As4)(LA)] (LA = Fe(CO)4 (4); B(C6F5)3 (7)) and [Cp″2Zr(µ,η3:1:1-As4)(Fe(CO)3)] (5) or a di-substitution [Cp″2Zr(µ3,η1:1:1:1-As4)(LA)2] (LA = W(CO)5 (2); CpMn(CO)2 (3); AlR3 (6, R = Me, Et, iBu)) are monitored. In contrast to other coordination products, 5 shows an η3 coordination in which the butterfly As4 ligand is rearranged to a cyclo-As4 ligand. The reported complexes are rationalized in terms of inverse coordination.


Introduction
The interest in the activation of small molecules, such as H 2 , N 2 , NH 3 , etc., has increased notably in recent years [1][2][3][4][5][6][7][8]. A special focus lies on the activation of cage compounds such as white phosphorus and yellow arsenic by transition metals and main group compounds [9][10][11][12]. While the synthesis and isolation of polyphosphorus complexes have been intensively investigated [9][10][11], polyarsenic complexes are much less known, and hence the number of comparable polyarsenic complexes is quite limited [12]. This can be attributed to the very challenging handling of As 4 due to its pronounced air-and light-sensitivity, time-consuming preparation, and to the lack of knowledge regarding its toxicitiy. Nevertheless, by now, several examples of As n -containing main group compounds and transition metal complexes are known [12]. Selected representatives are depicted in Scheme 1. A remarkable example is the reaction of yellow arsenic towards the Cp PEt radical (Cp PEt = C 5 (4-EtC 6 H 4 ) 5 ), which leads to the formation of the first organo-substituted As 4 butterfly compound [Cp Pet 2 As 4 ] (I, Scheme 1) [13]. Furthermore, the reaction of the silylene [PhC(N t Bu) 2 SiN(SiMe 3 ) 2 ] with yellow arsenic results in the aggregation of the As 4 tetrahedron and compound II that contains a heptaarsa-nortricyclane unit can be isolated [14]. Surprisingly, by using the disilene [Cp*(Me 3 Si) 2 NSi = SiN(Me 3 Si) 2 Cp*] (Cp* = C 5 Me 5 ), the butterfly-like compound [Cp*{(SiMe 3 ) 2 N}SiAs] 2 (III, Scheme 1) is formed [14].
Compared to the As-containing main group compounds, As n ligand complexes of transition metals have been investigated in more detail and show a more extensive chemistry. Dahl et. al. described the first As n ligand complex [(CO) 3 Co(η 3 -As 3 )] (IV, Scheme 1) in the late 1960s obtained by the reaction of [Co 2 (CO) 8 ] with [AsCH 3 ] 5 [15]. Many of the Scheme 1) in the late 1960s obtained by the reaction of [Co2(CO)8] with [AsCH3]5 [15]. Many of the described syntheses use in situ prepared yellow arsenic and transition metal compounds with labile ligands [12]. For instance, the photolysis of [Cp*Nb(CO)4] in the presence of yellow arsenic leads to the formation of V, containing a tetraarsacyclobutadiene ligand [16]. Another remarkable example is the pentaarsaferrocene [Cp*Fe(η 5 -As5)] (VI, Scheme 1) [17]. After its discovery by Scherer et al. in 1990, a versatile chemistry emerged [18][19][20][21][22][23], including, inter alia, redox- [19,20] and coordination chemistry [21][22][23]. Recently, our group reported the synthesis of [Cp''2Zr(η 1:1 -As4)] (1, Scheme 1) by the thermolysis of As4 with [Cp''2Zr(CO)2] (Cp'' = 1,3di-tertbutyl-cyclopentadienyl) [24]. Further investigations illustrate the high potential of 1 as an arsenic transfer reagent [24][25][26]. Additionally, the phosphorus congener of 1, i.e., [Cp''2Zr(η 1:1 -P4)], shows diverse reactivity patterns towards Lewis acids [27]. These results led us to investigate the reaction behavior of 1 towards the same Lewis acids to possibly functionalize the As4 core in 1. That way, compounds with a higher molecular mass should be accessible, resulting, in the case of a transfer of the functionalized As4 ligand, in a better solubility of the products. Herein, we report the coordination behavior of 1 towards different Lewis-acidic transition metal complexes and main group compounds, leading among others to hetero bi-and trimetallic complexes. The structure of these complexes can be rationalized also by means of the concept of inverse coordination [28][29][30][31][32][33]. Within this concept, the structure of complexes is rationalized in the sense that the ligand represents the central entity to which the metal centers are connected. In this type of complex, the distribution of donor and acceptor sites is opposed to the conventional complexes.

General Consideration
To obtain a better insight into the electronic structure and to determine the favored coordination site of 1 to Lewis acids, DFT calculations were carried out. The frontier molecular orbitals of 1 at the B3LYP/def2TZVP level of theory are depicted in Figure 1 (see also Supplementary Materials). The highest occupied molecular orbital (HOMO) represents the Zr-As bonding, while the lone pairs of electrons of the bridgehead and wingtip arsenic atoms (arsenic atoms bonded to Zr) are the HOMO-1 and HOMO-2, respectively. Both are energetically close to each other, but the HOMO-2 has a considerably higher arsenic atomic orbital contribution from the wingtip arsenic atoms (64%) compared to the HOMO-1 orbital (26% for the wing tip and 34% from the bridgehead As atoms). This would favor a more effective orbital overlap of the wingtip atoms with a potential acceptor orbital of a Lewis acid. Furthermore, the natural charge distribution shows a negative charge concentration on the wingtip arsenic atoms (nat. charge: −0.218) compared to the bridgehead arsenic atoms (nat. charge: +0.016; Figure 2). Herein, we report the coordination behavior of 1 towards different Lewis-acidic transition metal complexes and main group compounds, leading among others to hetero bi-and trimetallic complexes. The structure of these complexes can be rationalized also by means of the concept of inverse coordination [28][29][30][31][32][33]. Within this concept, the structure of complexes is rationalized in the sense that the ligand represents the central entity to which the metal centers are connected. In this type of complex, the distribution of donor and acceptor sites is opposed to the conventional complexes.

General Consideration
To obtain a better insight into the electronic structure and to determine the favored coordination site of 1 to Lewis acids, DFT calculations were carried out. The frontier molecular orbitals of 1 at the B3LYP/def2TZVP level of theory are depicted in Figure 1 (see also Supplementary Materials). The highest occupied molecular orbital (HOMO) represents the Zr-As bonding, while the lone pairs of electrons of the bridgehead and wingtip arsenic atoms (arsenic atoms bonded to Zr) are the HOMO-1 and HOMO-2, respectively. Both are energetically close to each other, but the HOMO-2 has a considerably higher arsenic atomic orbital contribution from the wingtip arsenic atoms (64%) compared to the HOMO-1 orbital (26% for the wing tip and 34% from the bridgehead As atoms). This would favor a more effective orbital overlap of the wingtip atoms with a potential acceptor orbital of a Lewis acid. Furthermore, the natural charge distribution shows a negative charge concentration on the wingtip arsenic atoms (nat. charge: −0.218) compared to the bridgehead arsenic atoms (nat. charge: +0.016; Figure 2). The electrostatic potential (Figure 2, right) presents a similar picture, showing that the wingtip As atoms have the highest negative potential. This indicates that a coordination of the electron-rich arsenic atoms directly bound to the The electrostatic potential (Figure 2, right) presents a similar picture, showing that the wingtip As atoms have the highest negative potential. This indicates that a coordination of the electron-rich arsenic atoms directly bound to the zirconium atom should be favored. In contrast, due to the possible steric repulsion of the Cp'' ligand, a coordination of the bridgehead arsenic atoms could still be possible.  Recently, we investigated the coordination behavior of [Cp''2Zr(η 1:1 -P4)] towards Lewis-acidic metal fragments [27]. The results show that a coordination to Lewis acids via both the wing tip and bridgehead phosphorus atoms is possible, with the coordination via the wing tip P atoms being favored electronically but disfavored sterically. Since the single bond covalent radius of As is larger than that of P (1.21 Å vs. 1.11 Å for As and P, respectively) [34] and inherently the Zr-As bond (2.6619(4) Å to 2.6656(3) Å) [24] is longer compared to the Zr-P bond (2.5596(7) Å to 2.5620(6) Å) [27], one would expect that, for 1, the electronic effects should predominate and a coordination via wingtip arsenic atoms be preferred.

Coordination of 1 to Transition Metal Complexes
The reaction of 1 with Lewis-acidic transition metal compounds leads to single or double coordination of 1. Scheme 2 gives an overview of the reactivity of 1 towards transition metal compounds. All products were comprehensively characterized by mass spectrometry, NMR spectroscopy, IR spectroscopy, and single crystal X-ray structure analysis. The electrostatic potential (Figure 2, right) presents a similar picture, showing that the wingtip As atoms have the highest negative potential. This indicates that a coordination of the electron-rich arsenic atoms directly bound to the zirconium atom should be favored. In contrast, due to the possible steric repulsion of the Cp'' ligand, a coordination of the bridgehead arsenic atoms could still be possible.  Recently, we investigated the coordination behavior of [Cp''2Zr(η 1:1 -P4)] towards Lewis-acidic metal fragments [27]. The results show that a coordination to Lewis acids via both the wing tip and bridgehead phosphorus atoms is possible, with the coordination via the wing tip P atoms being favored electronically but disfavored sterically. Since the single bond covalent radius of As is larger than that of P (1.21 Å vs. 1.11 Å for As and P, respectively) [34] and inherently the Zr-As bond (2.6619(4) Å to 2.6656(3) Å) [24] is longer compared to the Zr-P bond (2.5596(7) Å to 2.5620(6) Å) [27], one would expect that, for 1, the electronic effects should predominate and a coordination via wingtip arsenic atoms be preferred.

Coordination of 1 to Transition Metal Complexes
The reaction of 1 with Lewis-acidic transition metal compounds leads to single or double coordination of 1. Scheme 2 gives an overview of the reactivity of 1 towards transition metal compounds. All products were comprehensively characterized by mass spectrometry, NMR spectroscopy, IR spectroscopy, and single crystal X-ray structure analysis. Recently, we investigated the coordination behavior of [Cp" 2 Zr(η 1:1 -P 4 )] towards Lewis-acidic metal fragments [27]. The results show that a coordination to Lewis acids via both the wing tip and bridgehead phosphorus atoms is possible, with the coordination via the wing tip P atoms being favored electronically but disfavored sterically. Since the single bond covalent radius of As is larger than that of P (1.21 Å vs. 1.11 Å for As and P, respectively) [34] and inherently the Zr-As bond (2.6619(4) Å to 2.6656(3) Å) [24] is longer compared to the Zr-P bond (2.5596(7) Å to 2.5620(6) Å) [27], one would expect that, for 1, the electronic effects should predominate and a coordination via wingtip arsenic atoms be preferred.

Coordination of 1 to Transition Metal Complexes
The reaction of 1 with Lewis-acidic transition metal compounds leads to single or double coordination of 1. Scheme 2 gives an overview of the reactivity of 1 towards transition metal compounds. All products were comprehensively characterized by mass spectrometry, NMR spectroscopy, IR spectroscopy, and single crystal X-ray structure analysis.  (5). By these reaction conditions, 4 is formed first, followed by CO elimination and the subsequent insertion of the Fe(CO)3 fragment into the As-As bond of the bridgehead As atoms (Scheme 2). The same reaction outcome, namely the formation of 5, can be observed when isolated 4 is heated to 70 °C in n-hexane for 2 h. This strongly indicates that 5 is formed from 4. A similar rearrangement of a P4-butterfly core has been reported for the reaction of [(Cp*Cr(CO)3}2(µ,η 1:1 -P4)] with [(Cr(CO)4(nbd)] (nbd = norbornadiene) [35]. In 2, 3 and 4, the As4 ligand in 1 serves as a four-valence electron donor, while, in 5, it serves as a six-electron donor.
DFT calculations at the B3LYP/def2-SVP level show that the coordination via the wingtip arsenic atoms is favored compared to the bridgehead arsenic atoms (Supplementary Materials). The coordination of one wing tip arsenic atom to one Lewis acid is exothermic with −58, −61 and −105 kJ·mol −1 for W(CO)5, CpMn(CO)2 and Fe(CO)4 fragments, respectively, while the coordination of a second fragment is similarly exothermic with −59, −68 and −100 kJ·mol −1 , respectively (Supplementary Table S5). These are in agreement with the experimental results, where in the reaction solution the formation of both mono and disubstituted species has been observed. In the case of 4 however, only the mono-substituted complex could be isolated, probably due to the lower solubility of 4 compared to the di-substituted derivative as well as the presence of a complex equilibrium in solution (vide infra).
DFT calculations at the B3LYP/def2-SVP level show that the coordination via the wingtip arsenic atoms is favored compared to the bridgehead arsenic atoms (Supplementary Materials). The coordination of one wing tip arsenic atom to one Lewis acid is exothermic with −58, −61 and −105 kJ·mol −1 for W(CO) 5 , CpMn(CO) 2 and Fe(CO) 4 fragments, respectively, while the coordination of a second fragment is similarly exothermic with −59, −68 and −100 kJ·mol −1 , respectively (Supplementary Table S5). These are in agreement with the experimental results, where in the reaction solution the formation of both mono and disubstituted species has been observed. In the case of 4 however, only the mono-substituted complex could be isolated, probably due to the lower solubility of 4 compared to the di-substituted derivative as well as the presence of a complex equilibrium in solution (vide infra).
Single crystals suitable for X-ray diffractions were obtained by storing a concentrated n-hexane (2) or n-pentane (3, 4, 5) solution at −78 • C. It should be mentioned that 5 always co-crystallizes with a certain amount of 4 in a ratio of approximately 10:1. The molecular structures in the solid state are depicted in Figure 3. In the compounds 2, 3 and 4, one or two arsenic atoms of the As 4 -butterfly moiety of 1 coordinate to the Lewis-acidic metal fragments. Due to the steric repulsion, the As 4 unit is slightly distorted. The As-As distances are still intact and in the range of a single bond (2: 2.4363(6) Å to 2.4649(6)Å; 3: 2.4394(4) Å to 2.4710(4) Å; 4: 2.4321(6) Å to 2.4791(4) Å) [13,16,19,24]. Furthermore, the Zr-As distances are slightly elongated compared to 1 [24]. In the case of 2 and 4, the As-W distances of 2.6560(4) Å and 2.6595(4) Å and the As-Fe distance of 2.4213(7) Å are slightly elongated compared to the sum of the single bond covalent radii (As-W: 2.58 Å; As-Fe: 2.37 Å) [34]. This indicates an elongated single bond between the coordinating As atoms and the metal center of the Lewis acids. In contrast, in 3, the As-Mn distances (2.3605(4) Å and 2.3732(5) Å) are slightly below the calculated value of 2.40 Å, which implies a single bond [34]. In 5, the As-As distance between the former bridgehead arsenic atoms is 3.032(3) Å, which clearly shows the cleavage of this bond. The other As-As bonds are still in the range of a single bond (2.4657(14) Å and 2.4721(13) Å) [13,16,19], and the As-Fe distances of 2.4151(16) Å to 2.4265 (15) Å are in the same range as observed for 4.
Molecules 2021, 26, x FOR PEER REVIEW 5 of 12 elongated compared to the sum of the single bond covalent radii (As-W: 2.58 Å; As-Fe: 2.37 Å) [34]. This indicates an elongated single bond between the coordinating As atoms and the metal center of the Lewis acids. In contrast, in 3, the As-Mn distances (2.3605(4) Å and 2.3732(5) Å) are slightly below the calculated value of 2.40 Å, which implies a single bond [34]. In 5, the As-As distance between the former bridgehead arsenic atoms is 3.032(3) Å, which clearly shows the cleavage of this bond. The other As-As bonds are still in the range of a single bond (2.4657(14) Å and 2.4721(13) Å) [13,16,19], and the As-Fe distances of 2.4151(16) Å to 2.4265 (15) Å are in the same range as observed for 4. The complexes 2-5 were characterized by IR and 1 H NMR spectroscopy as well as by mass spectrometry. While the expected carbonyl stretches were observed in the IR spectra of 2, 3, 4 and 5, marked differences are present in their 1 H NMR spectra. The 1 H NMR spectrum of 2 shows one set of signals for the Cp'' ligands (one singlet for the t Bu groups at 1.26 ppm and a triplet and a doublet at 5.31 ppm and 5.48 ppm, respectively, for the Cp ring-bound hydrogen atoms), while, in LIFDI-MS, a peak corresponding to [M + − W(CO)5] (corresponds to the mono-substituted complex) can be observed (Supplementary Materials). In the 1 H NMR spectrum of crystalline 3 dissolved in C6D6, three sets of signals corresponding to Cp'' ligands can be detected. In two of them, the adjacent CH groups are equivalent, indicating that the Cp'' ligands are in a symmetric environment, while, in the third set, they are not equivalent, pointing to an asymmetric compound ( Figure 4). One set of signals corresponding to a symmetric Cp'' ligand can be assigned to the starting material 1, the second set of signals for the symmetric Cp'' ligand is assigned to 3, based on the comparison with the 1 H NMR data of 2 which also shows a symmetric Cp'' ligand. The set of signals for the Cp'' ligands in an asymmetric environment can be attributed to [Cp''2Zr(µ,η 1:1:1 -As4){CpMn(CO)2}] (3′), based on the comparison with the 1 H NMR data for 4 ( Figure 4; vide infra). These data show that, in solution, 3 partly dissociates to 3′ and to 1 by elimination of one as well as both CpMn(CO)2 fragments. Attempts to freeze this dynamic process by lowering the temperature leads only to a temperature-dependent shift of the resonance signals as well as to a change of their relative intensity. By lowering the temperature, the intensity of the resonance signals corresponding to 3 increases and 3 crystallizes from the solution. These results are in stark contrast to the results reported for the related reaction of [Cp''2Zr(µ,η 1:1 -P4)] with [CpMn(CO)2thf], where the phosphorus analog of the mono-substituted 3′ and the coordination of one or both bridgehead phosphorus atoms to the manganese fragment were reported (see Figure S10 in Supplementary Materials) [27]. The complexes 2-5 were characterized by IR and 1 H NMR spectroscopy as well as by mass spectrometry. While the expected carbonyl stretches were observed in the IR spectra of 2, 3, 4 and 5, marked differences are present in their 1 H NMR spectra. The 1 H NMR spectrum of 2 shows one set of signals for the Cp" ligands (one singlet for the t Bu groups at 1.26 ppm and a triplet and a doublet at 5.31 ppm and 5.48 ppm, respectively, for the Cp ring-bound hydrogen atoms), while, in LIFDI-MS, a peak corresponding to [M + − W(CO) 5 ] (corresponds to the mono-substituted complex) can be observed (Supplementary Materials). In the 1 H NMR spectrum of crystalline 3 dissolved in C 6 D 6 , three sets of signals corresponding to Cp" ligands can be detected. In two of them, the adjacent CH groups are equivalent, indicating that the Cp" ligands are in a symmetric environment, while, in the third set, they are not equivalent, pointing to an asymmetric compound ( Figure 4). One set of signals corresponding to a symmetric Cp" ligand can be assigned to the starting material 1, the second set of signals for the symmetric Cp" ligand is assigned to 3, based on the comparison with the 1 H NMR data of 2 which also shows a symmetric Cp" ligand. The set of signals for the Cp" ligands in an asymmetric environment can be attributed to [Cp" 2 Zr(µ,η 1:1:1 -As 4 ){CpMn(CO) 2 }] (3 ), based on the comparison with the 1 H NMR data for 4 ( Figure 4; vide infra). These data show that, in solution, 3 partly dissociates to 3 and to 1 by elimination of one as well as both CpMn(CO) 2 fragments. Attempts to freeze this dynamic process by lowering the temperature leads only to a temperaturedependent shift of the resonance signals as well as to a change of their relative intensity. By lowering the temperature, the intensity of the resonance signals corresponding to 3 increases and 3 crystallizes from the solution. These results are in stark contrast to the results reported for the related reaction of [Cp" 2 Zr(µ,η 1:1 -P 4 )] with [CpMn(CO) 2 thf], where the phosphorus analog of the mono-substituted 3 and the coordination of one or both bridgehead phosphorus atoms to the manganese fragment were reported (see Figure S10 in Supplementary Materials) [27]. The 1 H NMR spectrum of isolated 4, dissolved in C6D6, shows one set of signals corresponding to Cp'' ligands in an asymmetric environment, two singlets at 1.15 ppm and 1.28 ppm for the t Bu groups and three triplets at 5.11 ppm, 5.22 ppm and 5.44 ppm for the CH groups. Therefore, no dynamic behavior in solution similar to that observed for 3 can be detected. In the reaction solution, however, among signals of 4, also signals corresponding to the di-substituted complex [Cp''2Zr(µ3,η 1:1:1:1 -As4){Fe(CO)4}2] (4′) and 1 can be detected by 1 H NMR spectroscopy (Supplementary Materials), but only 4 crystallizes from the solution. In the remaining solution, after crystallization of 4, three sets of signals can still be detected, even though the intensity of the signals of 4 is drastically decreased. Surprisingly, the 1 H NMR spectrum of 5 shows three signals (δ [ppm] = 1.32 (s), 5.84 (d), 5.95 (t)) for the Cp'' substituents indicating a symmetric environment on the NMR timescale (see Figure S14 in Supplementary Materials).

Coordination towards Main Group Compounds
In order to investigate whether the nature of the Lewis acid plays a crucial role in the coordination behavior of 1, the reactivity of 1 towards Lewis-acidic main group compounds was investigated (Scheme 3). Thus, the reaction with AlR3 (R = Me, Et, i Bu) in toluene was performed, leading to [Cp''2Zr(µ3,η 1:1:1:1 -As4)(AlR3)2] (6a: R = Me; 6b: R = Et; 6c: R = i Bu), showing twofold coordination of 1 to the AlR3 units. The reaction outcome is independent of the stoichiometry used, although the yield can be increased by using an excess of the Lewis acid. In contrast, the reaction with [B(C6F5)3] leads to the formation of [Cp''2Zr(µ3,η 1:1:1:1 -As4){B(C6F5)3}] (7). Probably due to the pronounced steric requirement of [B(C6F5)3], only the formation of the mono-substituted compound 7 seems to be possible, interestingly also at a wingtip arsenic atom. The less Lewis acidic Et3B do not reacts with 1.
DFT calculations support the reaction outcome (B3LYP/def2-SVP level of theory; for further information, see Supplementary Materials). In the case of the compounds 6 and 7, the coordination of the arsenic atoms directly bound to the zirconium is favored (−24 kJ·mol −1 (6a), −33 kJ·mol −1 (6b), −7 kJ·mol −1 (6c)) (see Supplementary Materials) as compared to the coordination of the bridgehead arsenic atoms. Furthermore, the formation of the di-substituted compounds 6a, 6b, and 6c is energetically clearly favored (6a: −28 The 1 H NMR spectrum of isolated 4, dissolved in C 6 D 6 , shows one set of signals corresponding to Cp" ligands in an asymmetric environment, two singlets at 1.15 ppm and 1.28 ppm for the t Bu groups and three triplets at 5.11 ppm, 5.22 ppm and 5.44 ppm for the CH groups. Therefore, no dynamic behavior in solution similar to that observed for 3 can be detected. In the reaction solution, however, among signals of 4, also signals corresponding to the di-substituted complex [Cp" 2 Zr(µ 3 ,η 1:1:1:1 -As 4 ){Fe(CO) 4 } 2 ] (4 ) and 1 can be detected by 1 H NMR spectroscopy (Supplementary Materials), but only 4 crystallizes from the solution. In the remaining solution, after crystallization of 4, three sets of signals can still be detected, even though the intensity of the signals of 4 is drastically decreased. Surprisingly, the 1 H NMR spectrum of 5 shows three signals (δ [ppm] = 1.32 (s), 5.84 (d), 5.95 (t)) for the Cp" substituents indicating a symmetric environment on the NMR timescale (see Figure S14 in Supplementary Materials).

Coordination towards Main Group Compounds
In order to investigate whether the nature of the Lewis acid plays a crucial role in the coordination behavior of 1, the reactivity of 1 towards Lewis-acidic main group compounds was investigated (Scheme 3). Thus, the reaction with AlR 3 (R = Me, Et, i Bu) in toluene was performed, leading to [Cp" 2 Zr(µ 3 ,η 1:1:1:1 -As 4 )(AlR 3 ) 2 ] (6a: R = Me; 6b: R = Et; 6c: R = i Bu), showing twofold coordination of 1 to the AlR 3 units. The reaction outcome is independent of the stoichiometry used, although the yield can be increased by using an excess of the Lewis acid. In contrast, the reaction with [B(C 6 F 5 ) 3 ] leads to the formation of [Cp" 2 Zr(µ 3 ,η 1:1:1:1 -As 4 ){B(C 6 F 5 ) 3 }] (7). Probably due to the pronounced steric requirement of [B(C 6 F 5 ) 3 ], only the formation of the mono-substituted compound 7 seems to be possible, interestingly also at a wingtip arsenic atom. The less Lewis acidic Et 3 B do not reacts with 1.
Single crystals suitable for X-ray diffractions can be obtained by storing a concentrated n-pentane (6a, 6b, 6c) or n-hexane (7) solution at −78 • C. The molecular structures of 6a and 7 are exemplified in Figure 5. In the case of the compounds 6, a dicoordination of 1 is observed, while, in 7, only the mono-coordination of 1 to boron occurs. The geometric parameters of 6a, 6b, and 6c are very similar, therefore, only 6a will be discussed further (for 6b, 6c see Supplementary Materials) herein. In 6a and 7, the As 4 -butterfly unit is only slightly distorted, which can be attributed to the steric repulsions. The As-As distances in 6a of 2.4272(4) Å to 2.4583(3) Å and 7 of 2.436(3) Å to 2.457(4) Å do not differ considerably from that reported for 1 [24]. However, the Zr-As distances are slightly elongated (6a: 2.6810(2) Å; 7: 2.732(3) Å) [24]. As in 2-4, the As-Al (6a: 2.6743(5) Å) and As-B (7: 2.177(6) Å) distances in 6a and 7 are slightly longer than the sum of the covalent radii (As-Al: 2.47 Å and As-B: 2.06 Å) [34].  Single crystals suitable for X-ray diffractions can be obtained by storing a concentrated n-pentane (6a, 6b, 6c) or n-hexane (7) solution at −78 °C. The molecular structures of 6a and 7 are exemplified in Figure 5. In the case of the compounds 6, a di-coordination of 1 is observed, while, in 7, only the mono-coordination of 1 to boron occurs. The geometric parameters of 6a, 6b, and 6c are very similar, therefore, only 6a will be discussed further (for 6b, 6c see Supplementary Materials) herein. In 6a and 7, the As4-butterfly unit is only slightly distorted, which can be attributed to the steric repulsions. The As-As distances in 6a of 2.4272(4) Å to 2.4583(3) Å and 7 of 2.436(3) Å to 2.457(4) Å do not differ considerably from that reported for 1 [24]. However, the Zr-As distances are slightly elongated (6a: 2.6810(2) Å; 7: 2.732(3) Å) [24]. As in 2-4, the As-Al (6a: 2.6743(5) Å) and As-B (7: 2.177(6) Å) distances in 6a and 7 are slightly longer than the sum of the covalent radii (As-Al: 2.47 Å and As-B: 2.06 Å) [34]. The 1 H NMR spectra of 6a, 6b, 6c, and 7 show the expected signals for the Cp'' ligands as well as the signals of the alkyl groups bonded to aluminum in compound 6. A doublet and a triplet corresponding to the hydrogen atoms directly bonded to the cyclopentadienyl ligands resonate between δ = 5.0 ppm to 5.78 ppm for 6. The corresponding signals for 7 are slightly more deshielded and resonate at 5.88 ppm and 6.06 ppm. Furthermore, a sharp singlet for the t Bu groups is observed (δ = 1.27 ppm (6a); 1.26 ppm (6b, 7); 1.29 ppm (6c)). Additionally, the expected signals for the alkyl groups of the aluminum Lewis acids appear, for 6a a singlet at −0.29 ppm (Supplementary Materials for 6b and 6c). The 1 H NMR spectra of 6a, 6b, 6c, and 7 show the expected signals for the Cp" ligands as well as the signals of the alkyl groups bonded to aluminum in compound 6. A doublet and a triplet corresponding to the hydrogen atoms directly bonded to the cyclopentadienyl ligands resonate between δ = 5.0 ppm to 5.78 ppm for 6. The corresponding signals for 7 are slightly more deshielded and resonate at 5.88 ppm and 6.06 ppm. Furthermore, a sharp singlet for the t Bu groups is observed (δ = 1.27 ppm (6a); 1.26 ppm (6b, 7); 1.29 ppm (6c)). Additionally, the expected signals for the alkyl groups of the aluminum Lewis acids appear, for 6a a singlet at −0.29 ppm (Supplementary Materials for 6b and 6c).

General Informations
All experiments were performed under an atmosphere of dry nitrogen or argon using Schlenk and glovebox techniques. Solvents were purified, dried and degassed prior to use. 1 H, 13 [36,37] and [Cp" 2 Zr(η 1:1 -As 4 )] [24] were prepared according to literature procedures.

)] (4)
A solution of [Cp" 2 Zr(η 1:1 -As 4 )] (1) (60 mg, 0.081 mmol) in 10 mL n-hexane is added to a suspension of [Fe 2 (CO) 9 ] (58 mg, 0.16 mmol) in 10 mL n-hexane. The reaction mixture is stirred at room temperature for 24 h. From the brown reaction mixture, the solvent was removed in vacuo. The brown residue was dissolved in 5 mL n-pentane and filtered via cannula. Orange crystals of 4 suitable for single crystal X-ray structure analysis were obtained by storing a concentrated solution at −78 • C. Crystalline yield: 11 mg (0.016 mmol, 20%). M.S. (Manfred Scheer), supervising the whole research work, writing of the paper. All authors have read and agreed to the published version of the manuscript.

Funding:
The Deutsche Forschungsgemeinschaft within the project Sche 384/32-2 supported this work.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding authors.