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 phosphinidene-bridged complexes [Mo 2 Cp(μ-κ 1 :κ 1 ,η 5 -PC 5 H 4 )(CO) 2 L] and [Mo 2 Cp 2 (μ-PH)-(CO) 2 L] (Cp = η 5 -C 5 H 5 ; L=η 6 -1,3,5-C 6 H 3 t Bu 3 ) react readily with [Fe 2 (CO) 9 ] or the solvate complexes [M(CO) 5 (THF)] (M=Cr, Mo, W; THF=tetrahydrofuran) to give, as a result of the addition of the corresponding 16-electron M(CO) n fragments to their multiple Mo-P bonds, the trinuclear derivatives [FeMo 2 Cp(μ 3 -κ 1 :κand [MMo 2 Cp 2 (μ 3 -PH)(CO) 7 L]. The PH-bridged complexes display a distorted tetrahedral environment around their P atom, according to X-ray diffraction studies on the Fe, Cr, and W compounds, whereas the PC 5 H 4 -bridged ones display an unusual trigonalpyramidal geometry around the P atoms (sum of M-P-M angles 359.6°for the Fe compound). The latter species were stable toward hydrolysis, whereas the trinuclear PH-bridged complexes underwent easy hydrolysis of their P-Mo(metallocene) bonds to give the corresponding PH 2 -bridged derivatives [FeMoCp(μ-PH 2 )(CO) 6 L] and [MMoCp(μ-PH 2 )(CO) 7 L] (M=Cr, Mo, W). § This paper is dedicated to the memory of Dr. J. Manuel Concell on, former dean of the School of Chemistry and distinguished professor of organic chemistry at the Universidad de Oviedo.

A method for the generation of transition metal-phosphorus multiple bonds has been developed using the reactions of a novel thiophosphinidene-bridged dimolybdenum complex with different metal carbonyls. The overall process could be considered as a transmetalation of the phosphinidene ligand involving the activation of P-S and P-Mo bonds.

The phosphinidene complex [Mo2Cp(micro-kappa1:kappa1,eta5-PC5H4)(CO)2(eta6-R*H)] (2; Cp = eta5-C5H5; R* = 2,4,6-C6H2tBu3) has substantially different Mo-P bonds and displays a high reactivity located at the short Mo-P bond. Sideways cycloaddition or addition processes are observed toward RCCR, HCl, and [Fe2(CO)9], to give respectively metallacyclobutene and arylphosphide-bridged and heterometallic phosphinidene-bridged derivatives, a behavior reminiscent of the nucleophilic mononuclear phosphinidene complexes (carbene-like behavior), which is in good agreement with the ground-state electronic structure of 2 derived from density functional theory calculations. However, the reaction of 2 with [Co2(CO)8] implies the addition of two cobalt fragments to its short Mo-P bond and thus reveals a carbyne-like behavior of compound 2. In most of the new products, the P atom displays an unprecedented trigonal-pyramidal-like environment, instead of the expected tetrahedral distribution of bonds.

The title phosphinidene complex reacted at room temperature with CS 2 and SCNPh to give the phosphanyl derivatives [Mo 2 Cp{- 2

The phosphide-bridged complex [Mo 2 Cp 2 (µ-P) (CO) In contrast, reaction with K[BH s Bu 3 ] at 213 K giVes the cyclopentadiene complex [Mo 2 Cp(µ-P)(CO) 2 (η 6 -HR*)(η 4 -C 5 H 6 )], resulting from hydride attack at a carbon atom. This compound undergoes spontaneous C-H bond cleaVage aboVe 253 K to giVe the phosphinidene complex [Mo 2 Cp 2 (µ-PH)(CO) 2 (η 6 -HR*)], which turns out to be the direct product of the reaction of 1 with Li[BHEt 3 ] in THF at 213 K. This thermally unstable species can be stabilized through reaction with BH 3 ‚THF, to giVe [Mo 2 Cp 2 {µ-κ 1 :κ 1 ,η 2 -HPBH 3 }(CO) 2 (η 6 -HR*)], the first complex reported to haVe the phosphinidene-borane (HPBH 3 ) ligand.

The isomers [Mo2Cp2(mu-kappa(1):kappa(1),eta(6)-PR*)(CO)2] (1) and [Mo2Cp(mu-kappa(1):kappa(1),eta(5)-PC5H4)(CO)2(eta(6)-HR*)] (2) (Cp = eta(5)-C5H5; R* = 2,4,6-C6H2(t)Bu3) react with [AuCl(THT)] and with the cation [Au(THT)2](+) (THT = tetrahydrothiophene) to give phosphinidene-bridged Mo2Au complexes resulting from the addition of an AuCl or Au(THT)(+) electrophile to their multiple P-Mo bonds. Removal of the Cl(-) or THT ligand from these derivatives causes a dimerization of the trinuclear structures to give the cationic derivative [{AuMo2Cp(mu3-kappa(1):kappa(1):kappa(1),eta(5)-PC5H4)(CO)2(eta(6)-HR*)}2](2+), which displays a novel H-shaped metal core held by strong Mo-Au dative bonds [2.768(1) A] and an aurophilic interaction [Au-Au = 3.022(1) A].