Isocyanide Cycloaddition and Coordination Processes at Trigonal Phosphinidene-Bridged MoRe and MoMn Complexes

: Heterometallic phosphinidene complexes are appealing species for the construction of novel organophosphorus ligands thanks to the high reactivity expected from the combination of M-P multiple bonding and the intrinsically different electronic and coordination preferences of the distinct metals. In a preliminary study, we found that the heterobimetallic complexMoReCp( µ -PMes*)(CO) 6 (Mes* = 2,4,6-C 6 H 2 t Bu 3 ) reacted with CN( p -C 6 H 4 OMe) via [2+1]-cycloaddition to form a novel azaphosphallene complex. We have now examined in detail the reactions of the above complex and those of its MoMn analogue with different isocyanides, which turned out to be strongly dependent on experimental conditions and on the size of the substituent at the isocyanide. All the products formed follow from one or several of the following reaction pathways: (i) CO substitution by CNR; (ii) addition of CNR at the group 7 metal centre; and (iii) [2+1] cycloaddition of isocyanide at a Mo=P bond to form azaphosphallene groups, with the former process being dominant in reactions at room temperature and for the Mn system. In contrast, low-temperature reactions of the Re system favoured the addition processes, with the [2+1] cycloaddition at Mo=P bonds only taking place at substrates without metal-metal bonds and when the size of the CNR group does not cause unbearable steric clashes when placed in between the Cp and Mes* groups.


Introduction
The chemistry of transition metal complexes bearing phosphinidene ligands has emerged as a captivating field of research, driven by their unique reactivity and potential applications in catalytic processes [1][2][3][4].When bound to just one metal atom, up to three extreme coordination modes have been identified for the phosphinidene group (A-C in Scheme 1), although reactivity studies have been developed so far only for complexes of types B and C, which in turn are known as electrophilic and nucleophilic complexes, respectively, due to similarities with the related carbene species [5].Yet, phosphinidenes are not restrained to coordinate to just one metal atom, and, in fact, numerous polynuclear complexes bearing bridging PR groups have been reported in the literature.Amongst them, binuclear complexes (D-G in Scheme 1) are particularly appealing species for the construction of novel organophosphorus groups through reactions with different small organic and inorganic molecules, since they typically retain a significant multiplicity in their metal-phosphorus bonds (excepting those compounds bearing a lone electron pair at the P atom, type F), combined with the potential cooperative effects that the two metal centres can induce [6].Almost all these studies have been focused so far exclusively on homometallic phosphinidene-bridged complexes, where two identical metals are present.However, in the search for new reactivity patterns for the phosphinidene ligands, an attractive avenue, which has been essentially neglected until recently, is the introduction of two distinct metals, aka heterometallic complexes.The rationale behind this approach stems from the expected changes that two different metals, each with a set of unique electronic properties and/or coordination preferences, could impart to the coordination and electronic properties of the phosphinidene group itself [7][8][9][10][11][12].In this area of work, we recently reported the high-yield synthesis of the heterobimetallic phosphinidene complexes [MoMCp(µ-PMes*)(CO) 6 ] (Mes* = 2,4,6-C 6 H 2 t Bu 3 ; M = Re (1a), Mn (1b)) [13][14][15].These complexes displayed unprecedented asymmetric bonding of the PR ligands, whereby the π-bonding interaction was largely located at the Mo-P bonds (G in Scheme 1), even when such localisation was not expected in terms of simple electron counting because the PR ligand is bridging over two isoelectronic 15-e − metal fragments.Obviously, the unprecedented coordination mode of the PR groups in compounds 1a,b raised the question as to whether this new bonding motif could be translated into new chemical reactivity for the PR group itself, and, hence, we started to study the chemical behaviour of these two compounds.These studies indicated that both complexes display a striking trend to undergo cycloaddition reactions involving their Mo-P double bonds in their reactions with unsaturated organic molecules, sometimes with singular results [13,16,17].For instance, while the reaction with alkynes and some heterocumulenes (i.e., S=C=NR) proceeds readily via formal [2+2] cycloaddition processes [16], those with molecules having N-N multiple bonds (diazoalkanes and azides) proceed rather via [2+1] cycloadditions [17], yielding in both cases new organophosphorus ligands, hence showcasing the utility of these MoMn and MoRe complexes for synthetic purposes.Our preliminary studies on the chemistry of compound 1a included the reaction with the isocyanide CN(p-C 6 H 4 OMe), which yielded, when performed at low temperature, the novel azaphosphallene complex [MoReCp{µ-η 2 P,C :κ 1 P -PMes*CN(p-C 6 H 4 OMe)}(CO) 6 {2κ-CN(p-C 6 H 4 OMe)}], following from an unconventional [2+1] cycloaddition of the isocyanide molecule to the Mo=P bond and additional CO/CNR ligand substitution [13].Additionally, we noticed that the newly formed azaphosphallene group in this compound displays an unprecedented coordination mode whereby it remains κ 1 P -bound to the Re atom while being η 2 P,C -bound to the second metal.This is in contrast with all previously reported coordination modes for these ligands in binuclear complexes, these including µ-κ 1 P :κ 1 P , µ-κ 1 P :κ 1 N or µ-κ 1 P :η 2 N,C coordination modes in W 2 [18] or Sc 2 [19] dimetallic complexes.Taking into consideration these findings, it was of interest to perform a more detailed study on the reactions of compounds 1a,b with various isocyanides under different reaction conditions, which is the purpose of the present paper.As will be shown below, our study unveils the existence of three distinct processes occurring in these reactions, namely CNR addition, CO-substitution, and CNR cycloaddition, with the interplay between these processes being highly influenced by the substituent of the incoming isocyanide and/or the reaction temperature and stoichiometry.
Inorganics 2023, 11, x FOR PEER REVIEW 2 of 17 two distinct metals, aka heterometallic complexes.The rationale behind this approach stems from the expected changes that two different metals, each with a set of unique electronic properties and/or coordination preferences, could impart to the coordination and electronic properties of the phosphinidene group itself [7][8][9][10][11][12].In this area of work, we recently reported the high-yield synthesis of the heterobimetallic phosphinidene complexes [MoMCp(μ-PMes*)(CO)6] (Mes* = 2,4,6-C6H2 t Bu3; M = Re (1a), Mn (1b)) [13][14][15].These complexes displayed unprecedented asymmetric bonding of the PR ligands, whereby the π-bonding interaction was largely located at the Mo-P bonds (G in Scheme 1), even when such localisation was not expected in terms of simple electron counting because the PR ligand is bridging over two isoelectronic 15-e -metal fragments.Obviously, the unprecedented coordination mode of the PR groups in compounds 1a,b raised the question as to whether this new bonding motif could be translated into new chemical reactivity for the PR group itself, and, hence, we started to study the chemical behaviour of these two compounds.These studies indicated that both complexes display a striking trend to undergo cycloaddition reactions involving their Mo-P double bonds in their reactions with unsaturated organic molecules, sometimes with singular results [13,16,17].
For instance, while the reaction with alkynes and some heterocumulenes (i.e., S=C=NR) proceeds readily via formal [2+2] cycloaddition processes [16], those with molecules having N-N multiple bonds (diazoalkanes and azides) proceed rather via [2+1] cycloadditions [17], yielding in both cases new organophosphorus ligands, hence showcasing the utility of these MoMn and MoRe complexes for synthetic purposes.Our preliminary studies on the chemistry of compound 1a included the reaction with the isocyanide CN(p-C6H4OMe), which yielded, when performed at low temperature, the novel azaphosphallene complex [MoReCp{μ-η 2 P,C:κ 1 P-PMes*CN(p-C6H4OMe)}(CO)6{2κ-CN(p-C6H4OMe)}], following from an unconventional [2+1] cycloaddition of the isocyanide molecule to the Mo=P bond and additional CO/CNR ligand substitution [13].Additionally, we noticed that the newly formed azaphosphallene group in this compound displays an unprecedented coordination mode whereby it remains κ 1 P-bound to the Re atom while being η 2 P,Cbound to the second metal.This is in contrast with all previously reported coordination modes for these ligands in binuclear complexes, these including μ-κ 1 P:κ 1 P, μ-κ 1 P:κ 1 N or μκ 1 P:η 2 N,C coordination modes in W2 [18] or Sc2 [19] dimetallic complexes.Taking into consideration these findings, it was of interest to between these processes being highly influenced by the substituent of the incoming isocyanide and/or the reaction temperature and stoichiometry.
Scheme 1. Coordination modes of PR ligands at mono-and bi-nuclear complexes.
Scheme 1. Coordination modes of PR ligands at mono-and bi-nuclear complexes.In this product, no displacement of CO has taken place; instead, one of the molecules of the reagent has been added at the Re centre while the second one has been incorporated at the former Mo=P bond via a formal [2+1]-cycloaddition reaction (see below).The formation of products involving the incorporation of two molecules of the reagent raised the question as to whether compounds of type 2 were precursors in their formation.In fact, this is the case for compounds of type 3, since the addition of a second equivalent of isocyanide to the mixtures obtained after the first addition completed the 2 → 3 transformation.Considering all these observations, it seems clear that these reactions take place initially through the substitution of a carbonyl ligand by the isocyanide, a process then followed mainly by the addition of a second molecule of the reagent selectively at the Re centre, then causing the scission of the intermetallic bond.Unfortunately, reactions of the manganese analogue 1b with different isocyanides under similar conditions showed very little selectivity, and although complexes of type 2 were identified in the crude reaction mixtures, they could not be separated nor fully characterised.

Reactions of Compounds 1a,b with Isocyanides: Stoichiometric Reactions
Reactions of compound 1a with stoichiometric amounts of CN t Bu or CN(p-C6H4OMe) take place rapidly at room temperature to mainly produce the corresponding CO-substitution products of formulae [MoReCp(μ-PMes*)(CO)5(2κ-CNR)] [R = p-C6H4OMe (2a.1), t Bu (2a.2)] (Scheme 2).These compounds are invariably accompanied by smaller amounts of a second type of product now following from CO-substitution and further addition of the reagent, both taking place selectively at the Re centre to form the bis(isocyanide) derivatives [MoReCp(μ-PMes*)(CO)5(2κ-CNR)2] [R = p-C6H4OMe (3a.1), t Bu (3a.2)].In the particular case of CN(p-C6H4OMe), a third type of product, also derived from the reaction with two equivalents of the reagent, could be identified in the corresponding reaction mixtures, although only in trace amounts, this being the azaphosphallene complex [MoReCp{μ-η 2 P,C:κ 1 P-PMes*CN(p-C6H4OMe)}(CO)6{2κ-CN(p-C6H4OMe)}] (4a.1).In this product, no displacement of CO has taken place; instead, one of the molecules of the reagent has been added at the Re centre while the second one has been incorporated at the former Mo=P bond via a formal [2+1]-cycloaddition reaction (see below).The formation of products involving the incorporation of two molecules of the reagent raised the question as to whether compounds of type 2 were precursors in their formation.In fact, this is the case for compounds of type 3, since the addition of a second equivalent of isocyanide to the mixtures obtained after the first addition completed the 2 → 3 transformation.Considering all these observations, it seems clear that these reactions take place initially through the substitution of a carbonyl ligand by the isocyanide, a process then followed mainly by the addition of a second molecule of the reagent selectively at the Re centre, then causing the scission of the intermetallic bond.Unfortunately, reactions of the manganese analogue 1b with different isocyanides under similar conditions showed very little selectivity, and although complexes of type 2 were identified in the crude reaction mixtures, they could not be separated nor fully characterised.Spectroscopic data for compounds 2 (see Section 3 and Table 1) are similar to each other and consistent with the structure shown on Scheme 2, this being reminiscent of that of the parent phosphinidene 1a after replacing one CO ligand with a CNR group at the Re centre.Accordingly, the IR spectra of these complexes display now weak high energy bands due to the terminally coordinated isocyanide ligands [υCN = 2149 (2a.1) and 2167 (2a.2) cm −1 ], these being accompanied by four C-O stretching bands, with the most energetic ones (ca.2010 cm −1 ) displaying a high intensity, which denote the presence of pyramidal Re(CO)3 oscillators in these molecules [20], hence a positioning the CNR ligand cis to the PMes* ligand.In agreement with this, five carbonyl resonances are clearly identified in the respective 13 C NMR spectra at room temperature, with three of them arising from Re-bound carbonyls, of which two display significant two-bond P-couplings, a circumstance also observed for the parent compound 1a, hence pointing to a similar coordination around the Re atom.Finally, the 31 P NMR spectra of these complexes display highly Scheme 2. Reactions of compound 1a with isocyanides under stoichiometric conditions.Spectroscopic data for compounds 2 (see Section 3 and Table 1) are similar to each other and consistent with the structure shown on Scheme 2, this being reminiscent of that of the parent phosphinidene 1a after replacing one CO ligand with a CNR group at the Re centre.Accordingly, the IR spectra of these complexes display now weak high energy bands due to the terminally coordinated isocyanide ligands [υ CN = 2149 (2a.1) and 2167 (2a.2) cm −1 ], these being accompanied by four C-O stretching bands, with the most energetic ones (ca.2010 cm −1 ) displaying a high intensity, which denote the presence of pyramidal Re(CO) 3 oscillators in these molecules [20], hence a positioning the CNR ligand cis to the PMes* ligand.In agreement with this, five carbonyl resonances are clearly identified in the respective 13 C NMR spectra at room temperature, with three of them arising from Re-bound carbonyls, of which two display significant two-bond P-couplings, a circumstance also observed for the parent compound 1a, hence pointing to a similar coordination around the Re atom.Finally, the 31 P NMR spectra of these complexes display highly deshielded resonances (ca.700 ppm), as expected from complexes having phosphinidene ligands bridging over M-M bonds, as also found for the parent complexes 1a,b.The structure of compounds of type 3a can be derived from that of corresponding heptacarbonyls [MoMCp(µ-PMes*)(CO) 7 ] [15], after the replacement of two carbonyls at the Re atom with isocyanide ligands.Thus, the IR spectrum of these complexes now displays two high energy C-N bands and five C-O stretches (see Section 3 and Table 1) that are consistent with the presence of essentially independent Mo(CO) 2 and Re(CO) 3 (CNR) 2 oscillators, an assumption otherwise consistent with the absence of a Mo-Re bond connecting them, as also observed for the isoelectronic complex [MoReCp{µ- . As crystallographically confirmed for the latter complex, the two CNR groups display a mutually cisoid disposition, which is reflected in the similar intensities of the corresponding C-N stretches observed in the IR spectrum, while the high intensity of the most energetic C-O stretches (ca.2030 cm −1 ) denotes a facial arrangement of the three Re-bound carbonyls [20].Another spectroscopic indication of the lack of metal-metal interaction in compounds 3 arises from the 31 P NMR spectra, which for both complexes now display significantly less deshielded resonances (ca.530 ppm) compared to the metal-metal bonded complexes 2, a circumstance also observed when comparing the related hexa-and hepta-carbonyl complexes [MoMCp(µ-PMes*)(CO) n ] (n = 6, 7; M = Mn, Re) [15].Other spectroscopic data for these complexes are as expected and require no further discussion.resonances (ca.530 ppm) compared to the metal-metal bonded complexes 2, a circumstance also observed when comparing the related hexa-and hepta-carbonyl complexes [MoMCp(μ-PMes*)(CO)n] (n = 6, 7; M = Mn, Re) [15].Other spectroscopic data for these complexes are as expected and require no further discussion.

Reactions of Compounds 1a,b with Isocyanides: Reactions in the Presence of Excess CNR
Reactions of compounds 1a,b with excess CNR were found to be particularly complex, with the outcome being highly dependent on the size of the R group in the added reagent as well as on the temperature and the group 7 metal present on the parent phosphinidene.
Low-temperature (273 K) reactions of 1a with three equivalents of CN(p-C6H4OMe) led to the slow and progressive formation of the azaphosphallene complex [MoReCp{μη 2 P,C:κ 1 P-PMes*CN(p-C6H4OMe)}(CO)6{2κ-CN(p-C6H4OMe)}] (4a.1) (Scheme 3), following from the addition of two molecules of the reagent without decarbonylation, one of them added at the Re centre and the second one over the former Mo=P bond, to build an azaphosphallene ligand after a formal [2+1]-cycloaddition reaction.Although this compound could be isolated and fully characterized, its solutions evolved slowly at room temperature by elimination of the P-bound isocyanide group to give the phosphinidene complex [MoReCp(μ-PMes*)(CO)6{2κ-CN(p-C6H4OMe)}] (5a.1) (Scheme 3).This elimination process could be significantly accelerated upon heating (ca. 3 h at 333 K) and could be fully reversed upon addition of fresh isocyanide to solutions of 5a.1 to regenerate the azaphosphallene complex 4a.1.The temperature also has a significant influence on the outcome of these reactions, since the addition of excess CN(p-C6H4OMe) to 1a at room temperature led instead to the formation of a nearly 1:1 mixture of the azaphosphallene complexes 4a.1 and [MoReCp{μ-η 2 P,C:κ 1 P-PMes*CN(p-C6H4OMe)}(CO)5{2κ-CN(p- C6H4OMe)}2] (6a.1) (Scheme 3).The latter follows, in a formal sense, from an additional CO/CNR ligand substitution at 4a.1.However, independent experiments proved that such transformation (4a.1 + CNR → 6a.1) does not take place under the relevant reaction conditions, hence discarding 4a.1 as the precursor of the azaphosphallene 6a.1 during the course of these reactions.Instead, our experiments proved that the formation of 6a.1 rather follows from the addition of a third molecule of CNR to the phosphinidene 3a.1, which undergoes cycloaddition with the Mo=P bond of the complex, thus generating the new azaphosphallene ligand (Scheme 3).Scheme 3. Reactions of compound 1a with excess CNR.
In contrast with the chemistry just described, the addition of two equivalents of CN t Bu to solutions of 1a at 273 K did not yield any azaphosphallene product.Instead, a Scheme 3. Reactions of compound 1a with excess CNR.
In contrast with the chemistry just described, the addition of two equivalents of CN t Bu to solutions of 1a at 273 K did not yield any azaphosphallene product.Instead, a mixture of the addition product [MoReCp(µ-PMes*)(CO) 6 (2κ-CN t Bu)] (5a.2) and the corresponding addition plus substitution product 3a.2 were obtained invariably (Scheme 3).Furthermore, in this case, increasing the temperature did not substantially modify the outcome of the reaction since similar mixtures were then obtained, with the only difference now being the formation of small amounts of the CO-substitution product 2a.2, as discussed above.
Keeping in mind all these transformations, it seemed likely that compounds of type 5, formed by the simple addition of an isocyanide molecule to the parent phosphinidene 1a, could be the actual precursors of the azaphosphallene complexes 4, much in the same way as the phosphinidene complex 3a.1 is the precursor of the azaphosphallene 6a.1.To verify this hypothesis, we carried out some cross-experiment reactions between different isocyanides and compounds 5. Gratifyingly, compound 5a. 1  3), i Pr (7a.4)] (Scheme 4) in a completely selective way.However, this reaction failed for the isocyanide bearing a bulky t Bu substituent (CN t Bu), and in the case of the i Pr-substituted product 7a.4,we found that this complex was only of moderate stability, releasing the added CN i Pr group easily upon solvent removal.Both observations point to an important steric restriction on the nature of the CNR substituent in these cycloaddition reactions so that isocyanides bearing bulky groups such as those found in the alkyl isocyanides mentioned above would likely cause a significant steric clash when placed in between the Cp and Mes* groups as required to form the azaphosphallene groups, this explaining the absence of azaphosphallene products in the reaction of 1a with excess CN t Bu and the low stability of the i Pr derivative.Further corroborating the steric restrictions on the added CNR molecule, we found that the CN t Bu derivative 5a.corresponding addition plus substitution product 3a.2 were obtained invar 3).Furthermore, in this case, increasing the temperature did not substantia outcome of the reaction since similar mixtures were then obtained, w difference now being the formation of small amounts of the CO-substitution as discussed above.
Keeping in mind all these transformations, it seemed likely that comp 5, formed by the simple addition of an isocyanide molecule to the parent p 1a, could be the actual precursors of the azaphosphallene complexes 4, mu way as the phosphinidene complex 3a.1 is the precursor of the azaphosph verify this hypothesis, we carried out some cross-experiment reactions betw isocyanides and compounds 5. Gratifyingly, compound 5a.1 reacted easily isocyanides via [2+1] cycloadditions to give the corresponding aza complexes of formulae [MoReCp{μ-η 2 P,C:κ 1 P-PMes*CNR}(CO)6{2κ-CN(p-C6H p-C6H4OMe (4a.1), o-C6H4Me (7a.3), i Pr (7a.4)] (Scheme 4) in a completely However, this reaction failed for the isocyanide bearing a bulky t Bu substit and in the case of the i Pr-substituted product 7a.4,we found that this com of moderate stability, releasing the added CN i Pr group easily upon solvent observations point to an important steric restriction on the nature of the CN in these cycloaddition reactions so that isocyanides bearing bulky groups found in the alkyl isocyanides mentioned above would likely cause a sig clash when placed in between the Cp and Mes* groups as required azaphosphallene groups, this explaining the absence of azaphosphallene p reaction of 1a with excess CN t Bu and the low stability of the i Pr deriv corroborating the steric restrictions on the added CNR molecule, we found t derivative 5a.As was the case of the reactions of 1a with stoichiometric amounts of the isocyanide, the reactions of the manganese phosphinidene seem to follow a similar pathway, involving in the first place the substitution of CO by a CNR ligand.In this case, however, the putative intermediate retaining a Mo-Mn bond has not been observed during the reactions, likely due to the fast incorporation of the second CNR molecule to yield complex 3b.1.Obviously, the formation of the tris(isocyanide) complex 8b.1 would likely follow from an additional CO/CNR ligand substitution taking place at complex 3b.1 in the presence of excess reagent, a type of reaction not observed for the related Re complexes 3, surely a consequence of the well-known higher reluctance of the Re(CO) 3 fragment (compared to the Mn(CO) 3 one) to undergo dissociation of CO.During our preliminary exploration of the reactivity of 1a, we determined the solidstate structure of the azaphosphallene complex 4a.1 [13] (Figure 1), which confirmed the incorporation of two molecules of the isocyanide, one of them coordinated to the Re centre and displaying a perpendicular disposition with respect to the MoPRe plane, then rendering a nearly perfect octahedral coordination around the Re atom.The coordination of the second molecule of isocyanide can be visualised as arising from a [2+1] cycloaddition over the Mo=P double bond of the parent complex, which in consequence is significantly elongated to 2.544(1) Å (cf.2.274(1) Å in 1a [13]), and approaches the reference figures expected for single dative bonds (c.f.2.51 Å in [Mo(CO)5(PMe3)] [21]), as well as the figures measured for mononuclear iridium azaphosphallene complexes such as [IrCp*{η 2 P,C-PMes*(CNR)}(CNXyl)}] (ca.2.41 Å [22]), if we allow for the ca.0.13 Å difference in covalent radii of the metal atoms involved [23].The newly formed azaphosphallene group then acts as a bridging ligand in an unprecedented η 2 P,C:κ 1 P coordination mode, counterbalancing the quite different electron counts of the Mo and Re fragments (15 and 17 electrons, respectively).Thus, while the Re-P and P-C bonds display lengths close to the reference figures for conventional σ-bonds (ca.2.58 Å for Re-P and 1.80 Å for P-C [23]), the Mo-C separation is slightly shorter than the reference value of 2.27 Å for a Mo-C(sp 2 ) bond.In all, the azaphosphallene ligand contributes 4 electrons to the dimetal centre, giving a total electron count of 36 valence electrons for the complex.As a result, no metal-metal bond must be proposed according to the 18-electron rule, a circumstance in agreement with the long separation of 4.4675(5) Å between Mo and Re atoms in 4a.1.During our preliminary exploration of the reactivity of 1a, we determined the solidstate structure of the azaphosphallene complex 4a.1 [13] (Figure 1), which confirmed the incorporation of two molecules of the isocyanide, one of them coordinated to the Re centre and displaying a perpendicular disposition with respect to the MoPRe plane, then rendering a nearly perfect octahedral coordination around the Re atom.The coordination of the second molecule of isocyanide can be visualised as arising from a [2+1] cycloaddition over the Mo=P double bond of the parent complex, which in consequence is significantly elongated to 2.544(1) Å (cf.2.274(1) Å in 1a [13]), and approaches the reference figures expected for single dative bonds (c.f.2.51 Å in [Mo(CO) 5 (PMe 3 )] [21]), as well as the figures measured for mononuclear iridium azaphosphallene complexes such as [IrCp*{η 2 P,C -PMes*(CNR)}(CNXyl)}] (ca.2.41 Å [22]), if we allow for the ca.0.13 Å difference in covalent radii of the metal atoms involved [23].The newly formed azaphosphallene group then acts as a bridging ligand in an unprecedented η 2 P,C :κ 1 P coordination mode, counterbalancing the quite different electron counts of the Mo and Re fragments (15 and 17 electrons, respectively).Thus, while the Re-P and P-C bonds display lengths close to the reference figures for conventional σ-bonds (ca.2.58 Å for Re-P and 1.80 Å for P-C [23]), the Mo-C separation is slightly shorter than the reference value of 2.27 Å for a Mo-C(sp 2 ) bond.In all, the azaphosphallene ligand contributes 4 electrons to the dimetal centre, giving a total electron count of 36 valence electrons for the complex.As a result, no metal-metal bond must be proposed according to the 18-electron rule, a circumstance in agreement with the long separation of 4.4675(5) Å between Mo and Re atoms in 4a.1.During our preliminary exploration of the reactivity of 1a, we determined the solidstate structure of the azaphosphallene complex 4a.1 [13] (Figure 1), which confirmed the incorporation of two molecules of the isocyanide, one of them coordinated to the Re centre and displaying a perpendicular disposition with respect to the MoPRe plane, then rendering a nearly perfect octahedral coordination around the Re atom.The coordination of the second molecule of isocyanide can be visualised as arising from a [2+1] cycloaddition over the Mo=P double bond of the parent complex, which in consequence is significantly elongated to 2.544(1) Å (cf.2.274(1) Å in 1a [13]), and approaches the reference figures expected for single dative bonds (c.f.2.51 Å in [Mo(CO)5(PMe3)] [21]), as well as the figures measured for mononuclear iridium azaphosphallene complexes such as [IrCp*{η 2 P,C-PMes*(CNR)}(CNXyl)}] (ca.2.41 Å [22]), if we allow for the ca.0.13 Å difference in covalent radii of the metal atoms involved [23].The newly formed azaphosphallene group then acts as a bridging ligand in an unprecedented η 2 P,C:κ 1 P coordination mode, counterbalancing the quite different electron counts of the Mo and Re fragments (15 and 17 electrons, respectively).Thus, while the Re-P and P-C bonds display lengths close to the reference figures for conventional σ-bonds (ca.2.58 Å for Re-P and 1.80 Å for P-C [23]), the Mo-C separation is slightly shorter than the reference value of 2.27 Å for a Mo-C(sp 2 ) bond.In all, the azaphosphallene ligand contributes 4 electrons to the dimetal centre, giving a total electron count of 36 valence electrons for the complex.As a result, no metal-metal bond must be proposed according to the 18-electron rule, a circumstance in agreement with the long separation of 4.4675(5) Å between Mo and Re atoms in 4a.1.2).Data taken from reference [13].
Spectroscopic data for 4a.1 and for the closely related compounds 7a.3-5 are similar and fully consistent with the solid-state structure of compound 4a.1 (see Section 3 and Table 1), with the main differences, obviously, being derived from the particular isocyanide groups present in each case.Thus, all these complexes display an IR spectrum displaying seven C-N and C-O stretching bands, with the most energetic one corresponding to the terminal, linearly coordinated CNR groups, as also found for the isocyanide complexes 2 and 3.The remaining six bands are indicative of the presence of essentially independent Mo(CO) 2 and Re(CO) 4 oscillators, with the corresponding intensities being consistent with the presence of a cisoid arrangement of carbonyls in the first fragments (81.1 • in the solidstate structure of 4a.1) and of a distorted disphenoidal arrangement in the second ones [20].The most characteristic spectroscopic feature of all these complexes is the dramatic shielding of the azaphosphallene 31 P NMR resonances, which appear around −270 ppm, a decrease of nearly 900 ppm when compared to the parent phosphinidene 1a.Such strong shielding is, nevertheless, consistent with the presence of a significantly strained three-membered Mo-P-C ring, and the resulting shifts are significantly lower than those measured for Ir [22] and Pt [24] mononuclear azaphosphallene complexes with η 2 P,C -coordinated groups (δ P in the range −150 to −190 ppm).An opposite effect was observed for the 13 C NMR resonances of the MoPC ring carbon atoms in these compounds, which now display quite deshielded resonances (ca.218 ppm) with large one-bond couplings to P (ca.75 Hz), both figures remaining similar to those described for most of the abovementioned mononuclear azaphosphallene complexes.Similar comments can be made for the diagnostic azaphosphallene resonances of the bis(isocyanide) complex 6a.1 (δ P = −269.2ppm; δ C = 225.8ppm; J PC = 74 Hz).The main difference here is observed in the IR spectrum of this complex, which now displays two bands above 2100 cm −1 , instead of one, to be assigned as the corresponding C-N stretches of the two terminally coordinated isocyanide groups placed in a cisoid arrangement.The IR spectrum also supports the coordination of these two ligands at the Re atom due to the presence of C-O stretches compatible with essentially independent pyramidal Re(CO) 3 and Mo(CO) 2 oscillators.

Characterisation of Compounds 5
The molecule of the phosphinidene complex 5a.1 in the crystal (Figure 2 and Table 2) is comparable to that of the heptacarbonyl complexes [MoMCp(µ-PAr*)(CO) 7 ] (M = Re [14], Mn [15]), if we replace one of the carbonyl ligands of the M(CO) 5 fragments in the latter complexes by a CN(p-C 6 H 4 OMe) ligand.Thus, the molecule is built from MoCp(CO) 2 and Re(CO) 4 (CNR) fragments, expectedly joined by an asymmetrically bridging phosphinidene ligand of type E. As also observed for the above-mentioned heptacarbonyls, the Mo-P separation of 2.284(1) Å, is slightly longer than expected for a double bond interaction (cf.2.2212(8) Å in [MoCp(PClMes*)(CO) 2 ]) [13], while the Re-P distance of 2.581(1) Å remains consistent with the figures expected for an essentially single bond interaction (cf.2.571(2) Å for [Re(PPh 2 )(CO) 3 (N,N -phen)] [25]).In line with our previous hypotheses, these figures suggest that the π-bonding interaction of the PMes* ligand in 5a.1, and also in the related heptacarbonyls, is not particularly delocalised over the Mo-P-Re triangle; therefore, the origin of Mo-P elongation must be attributed mainly to the steric pressure created by the M(CO) 5-x (CNR) x (x = 0, 1) fragments in these complexes.As for the CNR group in 5a.1, we note that the Re-C distance (2.097(5) Å) is almost identical to the corresponding one determined for the azaphosphallene 4a.1, while the disposition of this ligand nearly perfectly bisects the Mo(CO) 2 fragment, surely to minimise the Mes*/CNR repulsions, hence virtually generating a non-crystallographic molecular symmetry plane containing the Mo-P-Re triangle.Spectroscopic data for compounds 5 are comparable to each other and totally consistent with the solid-state structure of 5a.1.The IR pattern of these complexes is almost identical to that of the azaphosphallene complexes 4a.1 and 7a.3-5 due to the presence in all cases of similar and vibrationally isolated Mo(CO)2 and M(CO)4(CNR) oscillators, although there is a clear difference in the position of the two less energetic bands in the spectrum, which mainly arise from the symmetric and antisymmetric stretches of the dicarbonylic fragment.In fact, these two bands are roughly 30 cm -1 lower in energy in compounds 5 when compared to those in 4a.1 or 7a.3-5, a difference surely caused by the higher electron-releasing properties of the phosphinidene ligand compared to the azaphosphallene one.Indeed, a similar effect can be observed when comparing the IR data for the phosphinidene/azaphosphallene pair 3a.1/6a.1.Finally, we note that the 31 P NMR spectra of these complexes are consistent with the presence of asymmetrically coordinated phosphinidene groups of type E, displaying moderately deshielded resonances (ca.480 ppm) as expected [6], while the abovementioned non-crystallographic symmetry plane explains the reduced number of signals observed in the 1 H and 13 C NMR spectra of these complexes.Spectroscopic data for compounds 5 are comparable to each other and totally consistent with the solid-state structure of 5a.1.The IR pattern of these complexes is almost identical to that of the azaphosphallene complexes 4a.1 and 7a.3-5 due to the presence in all cases of similar and vibrationally isolated Mo(CO) 2 and M(CO) 4 (CNR) oscillators, although there is a clear difference in the position of the two less energetic bands in the spectrum, which mainly arise from the symmetric and antisymmetric stretches of the dicarbonylic fragment.In fact, these two bands are roughly 30 cm −1 lower in energy in compounds 5 when compared to those in 4a.1 or 7a.3-5, a difference surely caused by the higher electron-releasing properties of the phosphinidene ligand compared to the azaphosphallene one.Indeed, a similar effect can be observed when comparing the IR data for the phosphinidene/azaphosphallene pair 3a.1/6a.1.Finally, we note that the 31 P NMR spectra of these complexes are consistent with the presence of asymmetrically coordinated phosphinidene groups of type E, displaying moderately deshielded resonances (ca.480 ppm) as expected [6], while the abovementioned non-crystallographic symmetry plane explains the reduced number of signals observed in the 1 H and 13 C NMR spectra of these complexes.

Characterisation of the Manganese Derivatives 3b.1 and 8b.1
The spectroscopic data for compound 3b.1 (see Section 3 and Table 1) are indicative of the close structural relationship between this compound and its Re analogues 3a; hence, a detailed description is not required here.The main spectroscopic difference arises from the change in metal, which is particularly evident in the 31 P NMR chemical shift of the phosphinidene group of compound 3b.1, some 30 ppm higher than that of its Re analogue 3a.1, a common observation when comparing analogous Re/Mn couples [26].As for the tris(isocyanide) complex 8b.1, its IR spectrum displays a set of seven C-N and C-O stretching bands, with those three above 2100 cm −1 being assigned to the C-N stretches of linearly coordinated CNR groups, while the relative intensities of these bands are indicative of a meridional disposition of these ligands around the Re atom, and the remaining four bands would arise from essentially uncoupled Re(CO) 2 and Mo(CO) 2 oscillators.Not unexpectedly, the IR features derived from the Re(CO) 2 (CNR) 3 fragment in 8b.1 are similar to those observed for the mononuclear complex [MnBr(CO) 2 (CNR) 3 ] [27].Further support for the proposed arrangement around the Re atom comes from the 13 C NMR spectrum, which displays just one resonance for the two Mo-bound carbonyls and another two signals for the Re-bound carbonyls, in agreement with the presence of a symmetry plane containing the metals and the P atom.Finally, the presence of an asymmetrically bridging phosphinidene ligand of type E in the molecule is supported by its moderately deshielded 31 P-NMR resonance, which is actually not very different from that of the phosphinidene 3b.1.

General Procedures and Starting Materials
All manipulations and reactions were carried out under an argon (99.995%) atmosphere using standard Schlenk techniques.Solvents were purified according to published procedures and distilled prior to use [28].Compounds 1a and 1b were prepared as described previously [13][14][15].All other reagents were obtained from the usual commercial suppliers and used as received unless otherwise stated.Petroleum ether refers to that fraction of distillation in the range 338-343 K. Chromatographic separations were carried out using jacketed columns refrigerated by a cryostat (ca.258 K).Aluminium oxide for chromatography (activity I, 70-290 mesh) was degassed under vacuum prior to use and then mixed under argon with the appropriate amount of water to reach activity IV.IR (Perkin Elmer, Waltham, MA, USA) stretching frequencies of CO and CNR ligands were measured in solution or in Nujol mulls (using CaF 2 windows in both cases) and are referred to as υ(CX)(solvent) or υ(CX)(Nujol), respectively.Nuclear magnetic resonance (NMR) (Bruker, Hamburg, Germany) spectra were routinely recorded at 300.13 ( 1 H), 121.49 ( 31 P{ 1 H}) or 100.63 MHz ( 13 C{ 1 H}) at 295 K, unless otherwise stated.Chemical shifts (δ) are given in ppm, relative to internal tetramethylsilane ( 1 H, 13 C) or external 85% aqueous H 3 PO 4 ( 31 P).Coupling constants (J) are given in Hertz.In the description of the NMR resonances, "bs" stands for broad singlet and "fd" for false doublet.Neat CN(p-C 6 H 4 OMe) (0.006 g, 0.044 mmol) was added to a solution of compound 1a (0.035 g, 0.044 mmol) in toluene (12 mL) and the mixture was stirred at room temperature for 2 h to give a brown solution.The solvent was then removed under vacuum, the residue extracted with dichloromethane/petroleum ether (1:8), and the extracts chromatographed on alumina at 258 K. Elution with the same solvent mixture gave a brown fraction containing small amounts of unreacted 1a, then a red fraction yielding, after removal of solvents, compound 2a.1 as a red microcrystalline solid (0.018 g, 45%).Elution with dichloromethane/petroleum ether (1:6) gave a green fraction containing small amounts of compound 3a.1, followed by a yellow fraction containing trace amounts of compound 4a.Neat CN t Bu (4 µL, 0.035 mmol) was added to a solution of compound 1a (0.025 g, 0.032 mmol) in toluene (8 mL) and the mixture was stirred at room temperature for 1.5 h to produce a red solution.The solvent was then removed under vacuum, the residue extracted with petroleum ether, and the extracts chromatographed on alumina at 258 K. Elution with the same solvent gave a brown fraction containing small amounts of unreacted 1a.Elution with dichloromethane/petroleum ether (1:20) gave a red fraction yielding, after removal of solvents, compound 2a.2 as a red microcrystalline solid (0.014 g, 52%  Neat CN(p-C 6 H 4 OMe) (0.004 g, 0.030 mmol) was added to a solution of compound 1a (0.029 g, 0.025 mmol) in toluene (5 mL) and the mixture was stirred at room temperature.Other two additions of neat CN(p-C 6 H 4 OMe) (0.004 g, 0.030 mmol) were made to the solution after 3 and 5 h, respectively, obtaining a green solution.The solvent was then removed under vacuum, the residue extracted with dichloromethane/petroleum ether (1:4), and the extracts chromatographed on alumina at 258 K. Elution with the same solvent mixture gave a green fraction, yielding, after removal of solvents, compound 3a.1 as a green microcrystalline solid (0.014 g, 54%).Anal.Calcd.for C 44 .5   Neat CN t Bu (12 µL, 0.106 mmol) was added to a solution of compound 1a (0.030 g, 0.038 mmol) in toluene (8 mL) and the mixture was stirred at room temperature for 1 h to give a green solution.The solvent was then removed under vacuum, the residue extracted with dichloromethane/petroleum ether (1:3), and the extracts chromatographed on alumina at 258 K. Elution with the same solvent mixture gave a brown fraction containing small amounts of unreacted 1a, then a second green fraction yielding, after removal of solvents, compound 3a.2 as a green microcrystalline solid (0.023 g, 66% Neat CN(p-C 6 H 4 OMe) (0.016 g, 0.120 mmol) was added to a solution of compound 1a (0.030 g, 0.038 mmol) in toluene (8 mL), and the mixture was stirred at 273 K for 3 h to give an orange solution.The solvent was then removed under vacuum, the residue extracted with dichloromethane/petroleum ether (1:2), and the extracts chromatographed on alumina at 258 K. Elution with the same solvent mixture gave first a green fraction containing small amounts of 3a.1, and then a yellow fraction yielding, after removal of solvents, compound 4a.1 as a yellow microcrystalline solid (0.030 g, 74%).The crystals of 4a.1 used in the X-ray diffraction study were grown by the slow diffusion of layers of petroleum ether and diethyl ether into a concentrated toluene solution of the complex at 253 K. Anal  A freshly prepared solution of compound 4a.1 (0.020 g, 0.019 mmol) in toluene (8 mL) was transferred into a Schlenk tube equipped with a Young valve, and the solution was heated at 333 K for 3 h to produce a green solution.The solvent was then removed under vacuum, the residue extracted with dichloromethane/petroleum ether (1:4), and the extracts chromatographed on alumina at 258 K. Elution with the same solvent mixture gave a green fraction, yielding, after removal of solvents, compound 5a.1 as a green microcrystalline solid (0.012 g, 71%).The crystals of 5a.1 used in the X-ray diffraction study were grown by the slow diffusion of a layer of petroleum ether into a concentrated dichloromethane solution of the complex at 253 K. Anal.Calcd.for C 37 H 41 MoReO  A solution of compound 1a (0.040 g, 0.051 mmol) in toluene (8 mL) was cooled down to 273 K, then neat CN t Bu (12 µL, 0.106 mmol) was added, and the mixture was stirred at 273 K for 9 h to give a green solution.The solvent was then removed under vacuum, the residue extracted with dichloromethane/petroleum ether (1:8), and the extracts chromatographed on alumina at 258 K. Elution with the same solvent mixture gave a green fraction, yielding, after removal of solvents, compound 5a.2 as a green microcrystalline solid (0.025 g, 53%).Elution with dichloromethane/petroleum ether (1:3) gave a green fraction yielding, after removal of solvents, compound 3a.2 as a green microcrystalline solid (0.010 g, 23%).Satisfactory elemental analysis of compound 5a. 2   Data collection for 5a.1 was performed at 153(4) K on an Oxford Diffraction Xcalibur Nova single crystal diffractometer, using Cu-K α radiation (λ = 1.5418Å) [29].Images were collected at a 62 mm fixed crystal-detector distance using the oscillation method with 1.00 • oscillation and variable exposure time per image (2.0-5.0 s).The data collection strategy was calculated with the program CrysAlis Pro CCD [30].Data reduction and cell refinement were performed with the program CrysAlis Pro RED [30].An empirical absorption correction was applied using the SCALE3 ABSPACK algorithm as implemented in the program CrysAlis Pro RED (Oxford Diffraction Ltd., Oxford, UK, 2006).Using the program suite WinGX [31], the structure was solved by Patterson interpretation and phase expansion using SHELXL2018/3 [32,33], and refined with full-matrix least squares on F 2 using the same software.Two t Bu groups and the methoxyphenyl group were found to be disordered.For the methoxyphenyl group, the best solution was obtained by modelling the disorder over two positions with 0.6/0.4occupancies.The disorder in two positions present in two of the t Bu groups was modelled by introducing six instead of three carbon atoms in each case with 0.6/0.4 and 0.5/0.5 occupancies, respectively.All non-hydrogen atoms were refined anisotropically, except for atoms involved in the modelled disorders, which were refined isotropically to prevent their temperature factors from becoming non-positive definite, causing the appearance of an A-level alert in the corresponding checkcif file.All hydrogen atoms were geometrically placed and refined using a riding model.CCDC 2283058 contains the supplementary crystallographic data for this paper.These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (accessed on 4 September 2023) (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336033; E-mail: deposit@ccdc.cam.ac.uk).

Conclusions
The reactions of the heterobimetallic phosphinidene-bridged complexes [MoMCp(µ-PMes*)(CO) 6 ] (Mes* = 2,4,6-C 6 H 2 t Bu 3 ; M = Re, Mn) with isocyanides are intricate, with the result depending critically on the reaction conditions (temperature and stoichiometry) as well as on the nature of the added isocyanide, particularly the steric demands of its substituent.Our experiments indicate that these reactions follow three possible pathways: (i) substitution of CO by CNR at the group 7 metal centre; (ii) addition of CNR at the group 7 metal centre and, iii) [2+1] cycloaddition of CNR at the Mo=P bond to form novel azaphosphallene ligands in the unprecedented µ-η 2 P,C :κ 1 P coordination mode.In particular, room temperature reactions are mainly dominated by CO/CNR ligand substitution yielding, in the case of the Re complex, phosphinidene derivatives of type [MoReCp(µ-PMes*)(CO) 5 (2κ-CNR)], which retain a Mo-Re bond.However, further addition of the incoming reagent is still a viable option even under stoichiometric conditions, leading to bis(isocyanide) phosphinidene derivatives of type [MoMCp(µ-PMes*)(CO) 5 (2κ-CNR) 2 ] with no metal-metal bond.In the case of the Mn system, the CO/CNR ligand substitution is even more favourable and the only products obtained are the bis-and tris(isocyanide) complexes [MoMnCp(µ-PMes*)(CO) 7-n (2κ-CNR) n ] (R = p-C 6 H 4 OMe, n = 2, 3).For the Re system, on the other hand, a lowering of the temperature partially suppresses the CO substitution processes, so that single or double addition products are preferentially obtained, with the second addition step taking place at the Mo=P bond of intermediates of type [MoReCp(µ-PMes*)(CO) 6 (2κ-CNR)] formed first, to give azaphosphallene complexes of type [MoReCp{µ-η 2 P,C :κ 1 P -PMes*CNR)}(CO) 6 (2κ-CNR)].The latter process is largely inhibited when the size of the CNR group would cause unbearable steric clashes if placed in between the Cp and Mes* groups (i.e., CN t Bu).In the same vein, we note that no cycloaddition reactions of the CNR ligands to the Mo=P bonds of the metal-metal-bound complexes 1 or their CNR-substituted derivatives seem to be possible in any case, perhaps due to the higher steric constraints derived from the closer approach of the metal fragments in these molecules.

Table 1 .
Selected IR 1 and 31 P{ 1 H} data 2 for new compounds.