New hydrido complexes of the type [M2Cp2(μ-H)(μ-PRR‘)(CO)4] (M = Mo, W) have been prepared through the thermal reaction of [Mo2Cp2(CO)6] with HPCy2, H2PCy, or HPEt2 or the thermal reaction of [W2Cp2(CO)4] with HPR2 (R = Cy, Et, Ph). In contrast, UV irradiation of [M2Cp2(CO)6] and HPRR‘ leads with good yield to the bis(phosphido) complexes [M2Cp2(μ-PRR‘)2(μ-CO)] (R = R‘ = Cy, Et, Ph; R = Cy, R‘ = H). Related complexes having different phosphido groups, [M2Cp2(μ-PR2)(μ-PR‘R‘ ‘)(μ-CO)] (R = Cy, tBu, Ph; R‘ = Cy, tBu, Et; R‘ ‘ = Cy, tBu, Et, H), can be prepared in high yield through the photochemical reaction of [M2Cp2(μ-PR2)(μ-H)(CO)4] and HPR‘R‘ ‘ or [M2Cp2(μ-H)(μ-PR‘R‘ ‘)(CO)4] and HPR2. All triply bonded compounds react easily with carbon monoxide at room temperature or under moderate heating to finally yield the corresponding trans-dicarbonyl complexes [M2Cp2(μ-PRR‘)2(CO)2] or [M2Cp2(μ-PR2)(μ-PR‘R‘ ‘)(CO)2]. Some of the intermediates in these carbonylation reactions have been identified, including the cis-dicarbonyl complex [Mo2Cp2(μ-PPh2)(μ-PtBu2)(CO)2] and the tricarbonyl complex [Mo2Cp2(μ-PEt2)2(CO)3]. The structures of the new complexes are analyzed on the basis of the corresponding IR and NMR (1H, 31P, 13C) data, and the reaction pathways operative in these highly efficient syntheses of bis(phosphido) complexes is discussed on the basis of the available data and some cross-experiments.

Chemical reduction of [Mo 2 Cp 2 (µ-Cl)(µ-PA 2 )-(CO) 2 ] (A ) Cy, Ph, OEt) gives the corresponding alkaline metal salts of the triply bonded anions [Mo 2 -Cp 2 (µ-PA 2 )(µ-CO) 2 ] -, which exhibit both molybdenum and oxygen nucleophilic sites. The PCy 2 anion reacts easily with NH 4 + , [AuCl(PR 3 )], or MeI to give unsaturated dicarbonyls [Mo 2 Cp 2 (µ-X)(µ-PA 2 )(CO) 2 ] (X ) H, AuPR 3 , Me), while [Me 3 O]BF 4 gives the methoxycarbyne [Mo 2 Cp 2 (µ-COMe)(µ-PCy 2 )(µ-CO)] and allyl chloride rearranges to give the unsaturated alkenyl complex [Mo 2 -Cp 2 (µ-PCy 2 )(µ-CMeCH 2 )(CO) 2 ].

The new cationic alkoxy and hydroxycarbyne complexes [M2Cp2(μ-COR)(μ-PR‘2)2]BF4 (Cp = η5-C5H5; M = W, R = Me, R‘ = Ph; M = Mo, R = Me and H, R‘ = Et) and [Mo2Cp2(μ-COR)(μ-COR‘)(μ-PCy2)]BF4 (R = Me; R‘ = H, Me, Et) are obtained in high yield by the reaction of the corresponding neutral monocarbonyl precursors with either [Me3O]BF4 or HBF4·OEt2 in dichloromethane. While the methoxycarbyne complexes are stable at room temperature, the analogous hydroxycarbyne species are thermally unstable, and above ca. 253 K they experience a hydrogen migration from oxygen to metal, to give the new hydride carbonyl complexes [Mo2Cp2(H)(μ-PEt2)2(CO)]BF4 and [Mo2Cp2(H)(μ-COMe)(μ-PCy2)(CO)]BF4 as major products. The electronic structure and bonding in some of these face-sharing bioctahedral complexes were studied by means of density functional theory. These calculations allow us to describe the intermetallic interaction present in the carbonyl-bridged 30-electron complexes by a configuration of type σ2δ4, with the δ orbitals being involved in π back-bonding to the carbonyl bridges. Upon methylation of the latter ligands, these δ-bonding orbitals become more delocalized over the Mo2C(carbyne) triangle and constitute the π-bonding component of the metal−carbyne bond. Therefore, a partial reduction of the direct metal−metal overlap occurs upon formation of these methoxycarbyne complexes, which still retain some multiplicity in the corresponding C−O bonds. The topological analysis of the electron density under the AIM scheme supports the above description of the Mo−Mo, Mo−C, and C−O bonds in these complexes.

The unsaturated complexes [W2Cp2(mu-PR2)(mu-PR'2)(CO)2] (Cp = eta5-C5H5; R = R' = Ph, Et; R = Et, R' = Ph) react with HBF4.OEt2 at 243 K in dichloromethane solution to give the corresponding complexes [W2Cp2(H)(mu-PR2)(mu-PR'2)(CO)2]BF4, which contain a terminal hydride ligand. The latter rearrange at room temperature to give [W2Cp2(mu-H)(mu-PR2)(mu-PR'2)(CO)2]BF4, which display a bridging hydride and carbonyl ligands arranged parallel to each other (W-W = 2.7589(8) A when R = R' = Ph). This explains why the removal of a proton from the latter gives first the unstable isomer cis-[W2Cp2(mu-PPh2)2(CO)2]. The molybdenum complex [Mo2Cp2(mu-PPh2)2(CO)2] behaves similarly, and thus the thermally unstable new complexes [Mo2Cp2(H)(mu-PPh2)2(CO)2]BF4 and cis-[Mo2Cp2(mu-PPh2)2(CO)2] could be characterized. In contrast, related dimolybdenum complexes having electron-rich phosphide ligands behave differently. Thus, the complexes [Mo2Cp2(mu-PR2)2(CO)2] (R = Cy, Et) react with HBF4.OEt2 to give first the agostic type phosphine-bridged complexes [Mo2Cp2(mu-PR2)(mu-kappa2-HPR2)(CO)2]BF4 (Mo-Mo = 2.748(4) A for R = Cy). These complexes experience intramolecular exchange of the agostic H atom between the two inequivalent P positions and at room-temperature reach a proton-catalyzed equilibrium with their hydride-bridged tautomers [ratio agostic/hydride = 10 (R = Cy), 30 (R = Et)]. The mixed-phosphide complex [Mo2Cp2(mu-PCy2)(mu-PPh2)(CO)2] behaves similarly, except that protonation now occurs specifically at the dicyclohexylphosphide ligand [ratio agostic/hydride = 0.5]. The reaction of the agostic complex [Mo2Cp2(mu-PCy2)(mu-kappa2-HPCy2)(CO)2]BF4 with CN(t)Bu gave mono- or disubstituted hydride derivatives [Mo2Cp2(mu-H)(mu-PCy2)2(CO)2-x(CNtBu)x]BF4 (Mo-Mo = 2.7901(7) A for x = 1). The photochemical removal of a CO ligand from the agostic complex also gives a hydride derivative, the triply bonded complex [Mo2Cp2(H)(mu-PCy2)2(CO)]BF4 (Mo-Mo = 2.537(2) A). Protonation of [Mo2Cp2(mu-PCy2)2(mu-CO)] gives the hydroxycarbyne derivative [Mo2Cp2(mu-COH)(mu-PCy2)2]BF4, which does not transform into its hydride isomer.

The triply bonded complex [Mo2Cp2(μ-H)-(μ-PCy2)(CO)2] (Cp = η5-C5H5) reacts readily at room temperature with a great variety of simple molecules, resulting in diverse processes, as illustrated by its reactions with CO (addition), CNtBu (insertion), and HSnPh3 (H2 elimination). This unsaturated hydride also easily incorporates 17e [MoCp(CO)3] or 16e [MnCp‘(CO)2] metal fragments to give 46e heterometallic clus-ters (Cp‘ = η5-C5H4Me).

The reactions of the phosphinidene-bridged complex [Mo(2)Cp(2)(μ-PH)(η(6)-HMes*)(CO)(2)] (1), the arylphosphinidene complexes [Mo(2)Cp(2)(μ-κ(1):κ(1),η(6)-PMes*)(CO)(2)] (2), [Mo(2)Cp(2)(μ-κ(1):κ(1),η(4)-PMes*)(CO)(3)] (3), [Mo(2)Cp(2)(μ-κ(1):κ(1),η(4)-PMes*)(CO)(2)(CN(t)Bu)] (4), and the cyclopentadienylidene-phosphinidene complex [Mo(2)Cp(μ-κ(1):κ(1),η(5)-PC(5)H(4))(η(6)-HMes*)(CO)(2)] (5) toward different sources of chalcogen atoms were investigated (Mes* = 2,4,6-C(6)H(2)(t)Bu(3); Cp = η(5)-C(5)H(5)). The bare elements were appropriate sources in all cases except for oxygen, in which case dimethyldioxirane gave the best results. Complex 1 reacted with the mentioned chalcogen sources at low temperature, to give the corresponding chalcogenophosphinidene derivatives [Mo(2)Cp(2){μ-κ(2)(P,Z):κ(1)(P)-ZPH}(η(6)-HMes*)(CO)(2)] (Z = O, S, Se, Te; P-Se = 2.199(2) Å). The arylphosphinidene complex 2 was the least reactive substrate and gave only chalcogenophosphinidene derivatives [Mo(2)Cp(2)(μ-κ(2)(P,Z):κ(1)(P),η(6)-ZPMes*)(CO)(2)] for Z = O and S (P-O = 1.565(2) Å), along with small amounts of the dithiophosphorane complex [Mo(2)Cp(2)(μ-κ(2)(P,S):κ(1)(S'),η(6)-S(2)PMes*)(CO)(2)], in the reaction with sulfur. The η(4)-complexes 3 and 4 reacted with sulfur and gray selenium to give the corresponding derivatives [Mo(2)Cp(2)(μ-κ(2)(P,Z):κ(1)(P),η(4)-ZPMes*)(CO)(2)L] (L = CO, CN(t)Bu), obtained respectively as syn (Z = Se; P-Se = 2.190(1) Å for L = CO) or a mixture of syn and anti isomers (Z = S; P-S = 2.034(1)-2.043(1) Å), with these diastereoisomers differing in the relative positioning of the chalcogen atom and the terminal ligand at the metallocene fragment, relative to the Mo(2)P plane. The cyclopentadienylidene compound 5 reacted with all chalcogens, and gave with good yields the chalcogenophosphinidene derivatives [Mo(2)Cp(μ-κ(2)(P,Z):κ(1)(P),η(5)-ZPC(5)H(4))(η(6)-HMes*)(CO)(2)] (Z = S, Se, Te), these displaying in solution equilibrium mixtures of the corresponding cis and trans isomers differing in the relative positioning of the cyclopentadienylic rings with respect to the MoPZ plane in each case. The sulfur derivative reacted with excess sulfur to give the dithiophosphorane complex [Mo(2)Cp(μ-κ(2)(P,S):κ(1)(S'),η(5)-S(2)PC(5)H(4))(η(6)-HMes*)(CO)(2)] (P-S = 2.023(4) and 2.027(4) Å). The structural and spectroscopic data for all chalcogenophosphinidene complexes suggested the presence of a significant π(P-Z) bonding interaction within the corresponding MoPZ rings, also supported by Density Functional Theory calculations on the thiophosphinidene complex syn-[Mo(2)Cp(2)(μ-κ(2)(P,S):κ(1)(P),η(4)-SPMes*)(CO)(3)].

The phosphinidene-bridged complex [Mo2Cp2(μ-PR*)(CO)4] (R = 2,4,6- C6H2 tBu3) experiences an intramolecular C−H bond cleavage from a tBu group to give the phosphide-hydride derivative [Mo2Cp2(μ-H){μ-P(CH2CMe2)C6H2 tBu2}(CO)4] in refluxing diglyme (ca. 438 K) or under exposure to near-UV−visible light. In contrast, its exposure to UV light yields two different dicarbonyl derivatives depending on the reaction conditions, either the triply bonded [Mo2Cp2(μ-PR*)(μ-CO)2] (Mo−Mo = 2.5322(3) Å) or its isomer [Mo2Cp2(μ-κ 1:κ 1,η 6-PR*)(CO)2], in which the phosphinidene ligand bridges asymmetrically the metal centers while binding its aryl group to one of the molybdenum atoms in a η6-fashion. The latter complex experiences a proton-catalyzed tautomerization to yield the cyclopentadienylidene−phosphinidene derivative [Mo2Cp(μ-κ 1:κ 1,η 5-PC5H4)(η 6-R*H)(CO)2]. Carbonylation of the η 6-phosphinidene complex proceeds stepwise through the η 4-tricarbonyl complex [Mo2Cp2(μ-κ 1:κ 1,η 4-PR*)(CO)3] and then to the starting tetracarbonyl compound, whereas its reaction with CNtBu yields only the η 4-complex [Mo2Cp2(μ-κ 1:κ 1,η 4-PR*)(CNtBu)(CO)2], which was characterized through an X-ray study. The η 4-tricarbonyl species reacts with CNtBu in tetrahydrofuran to give the metal−metal bonded derivative [Mo2Cp2(μ-PR*)(CNtBu)(CO)3]. In petroleum ether, however, this reaction yields the bis(isocyanide) derivative [Mo2Cp2(μ-PR*)(CNtBu)2(CO)3], which has an asymmetric trigonal phosphinidene bridge and no metal−metal bond. All the above results can be explained by assuming the operation of two primary processes in the photolysis of [Mo2Cp2(μ-PR*)(CO)4], one of them involving a valence tautomerization of the phosphinidene ligand, from the trigonal (four-electron donor) to the pyramidal (two-electron donor) coordination mode. The carbonylation reaction of the η 6-complex is accelerated by the presence of CuCl, due to the formation of the trimetal species [CuMo2(Cl)Cp2(μ-κ 1:κ 1:κ 1,η 6-PR*)(CO)2] and [CuMo2(Cl)Cp2(μ-κ 1:κ 1:κ 1,η 4-PR*)(CO)3]. The latter complexes were also characterized by single-crystal X-ray studies.

The reactions of the triply bonded anion [Mo2Cp2(μ-PCy2)(μ-CO)2]- (Li+ salt) with [NH4]PF6, MeI, and PhCH2Cl give, with good yields, the corresponding hydride- or alkyl-bridged derivatives [Mo2Cp2(μ-X)(μ-PCy2)(CO)2] (X = H, Me, CH2Ph). The related phenyl complex [Mo2Cp2(μ-Ph)(μ-PCy2)(CO)2] can be obtained upon reaction of the above anion with Ph3PbCl. According to the corresponding X-ray diffraction studies, the latter complex displays its phenyl group bonded to the dimetal center exclusively through the ipso carbon atom, while the methyl and benzyl complexes adopt an asymmetric α-agostic structure whereby one of the C−H bonds of the bridgehead carbon is bound to one of the molybdenum atoms. The intermetallic distances remain quite short in all cases, 2.56−2.58 Å. In solution, the hydride complex exhibits dynamic behavior involving mutual exchange of the carbonyl ligands. The alkyl derivatives behave similarly to each other in solution and also exhibit dynamic behavior, possibly implying the presence of small amounts of a nonagostic structure in equilibrium with the dominant α-agostic structure. Density functional theory calculations (B3LYP, B3PW91) correctly reproduce the experimental structures, and predict an α-agostic structure for both the methyl and benzyl complexes. The bonding in the above hydride and hydrocarbyl complexes was analyzed using molecular orbital, atoms in molecules, and natural bond orbital methodologies. The intermetallic binding in the hydride complex can be thus described as composed of a tricentric (Mo2H) plus two bicentric (Mo2) interactions, the latter being of σ and π types. In the hydrocarbyl-bridged complexes, analogous tricentric (Mo2C), and bicentric (Mo2) interactions can be identified, but there are additional interactions reducing the strength of the intermetallic binding, these being the α-agostic bonding in the case of the alkyl complexes and a π-donor interaction from the π-bonding orbitals of the hydrocarbon ring into suitable metal acceptor orbitals, in the case of the phenyl complex. The strength of these additional interactions have been estimated by second-order perturbation analysis to be of 70.3 (Me), 89.2 (CH2Ph), and 52.2 (Ph) kJ mol-1, respectively.