The metathesis reaction of [W2(OtBu)6] with tBuC⋮P leads to the phosphido complex [(tBuO)3W⋮P] (1), which can be stabilized when the reaction is carried out in the presence of Lewis acidic carbonyl complexes of the type [M(CO)5thf] (M = Cr, W). However, further reaction of the metathesis products [(tBuO)3W⋮CtBu] and [(tBuO)3W⋮P→M(CO)5] [M = W (4a), Cr (4b)] with tBuC⋮P leads after subsequent 1,3-OtBu shift to the metallaphosphacyclobutadiene derivatives [(tBuO)2W{C(tBu)}2P(OtBu)] (2a) and [(tBuO)2WC(tBu)P{M(CO)5}P(OtBu)] [M = W (5a), M = Cr (5b)]. To prevent the latter, the bulky phosphaalkyne Ar‘C⋮P (Ar‘ = 2,4,6-tBu3C6H2) was employed in the three-component reaction instead of tBuC⋮P. Surprisingly here, a “head-to-tail” dimerization of Ar‘C⋮P occurs to give the novel phosphinidene complexes [M(CO)4P(R‘)C(Ar‘)P{MCO)5}] [M = Cr (6a), M = W (6b); R‘ = 2,4-tBu2-7-Me2-indanyl]. In the case of [W(CO)5thf], additionally, the complex [W(CO)2{(η2-PCAr‘)W(CO)5}2] (7) is formed. The function of [W2(OtBu)6] in this reaction is uncertain, but stoichiometric amounts of [W2(OtBu)6] are essential to obtain complex 6. Reduction of the size of the alkoxide ligands by the use of [W2(ONp)6] (Np = CH2tBu) in the three-component reaction with Ar‘C⋮P leads to the trinuclear cluster compounds [W3〈(μ3-P){W(CO)5}〉(μ-ONp)3(ONp)6] (8) and [W3(μ3-P)(μ-ONp)3(ONp)6] (9). The crucial step of the side-product free synthesis of the phosphido complexes 4 is the introduction of MesC⋮P (Mes = 2,4,6-Me3C6H2) into the three-component system of [W2(OtBu)6], RC⋮P, and [W(CO)5thf]. Comprehensive structural and spectroscopic data are given for the products. Furthermore, a 31P NMR experiment was conducted following the reaction of [W2(OtBu)6] with MesC⋮P from −70 °C to ambient temperature. In the range of −60 to −40 °C, a singlet at 845 ppm with one pair of tungsten satellites (1 J W,P = 176 Hz) was observed for the complex [(tBuO)3W⋮P] (1).

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[Cr(CO)5PCl3] reacts with different metallates K[CpxMo(CO)3] [Cpx = ·5‐C5H4tBu (Cp]), ·5‐C5H3tBu2‐1,3 (Cp))], K[CpFe(CO)2], K[Cp]Cr(CO)3], K[C}*Ni(CO)] and Na2[Cr2(CO)10] at –78°C in THF to yiel various P2, cyclo‐ P3 and cyclo‐P5 ligand complexes of the formulae [Cp] Mo(CO)2 (.2‐P3)] (1a), [Cp]Mo(CO)2(.3‐P3)] (1b), [{CP'Mo‐(CO)2}2(μ,.2‐P2)] (2a), [{Cp]Mo(CO)2}2(μ, ·2‐P2)] (2b), [Cp]Fe).5‐P5)] (3), [(Cp'Cr)2(μ,.5‐P5)] (4), [{Cp]Cr(CO)2}2‐(μ,.2‐P2)] (5), [(Cp*Ni)2(μ,.2‐P2){Cr(CO)5}2] (6). [{(CO)5Cr}Cl2PPCl2{Cr(CO)5}] (7) and [{Cr(CO)5}2PCl (8). Comprehensive studies of the reaction pathway leading to formation of the cyclo‐P3 product 1a give strong indications that a sequence involving metal‐halogen exchange and stepwise P–P bond formation takes place, proceeding via [{CO}5Cr}Cl2PPCl2{Cr(CO)5}] (7) and the cyclo‐P3 precursor compond [{Cp]Mo(CO)3}{Cp'Mo(CO)2}2{Cr(CO)5}(μ,.3:1:1−P3)] (9). The latter two complexes have been isolated and structurally characterized.

Multiple PP bond formation in the coordination sphere of transition metals occurs in reactions of [Cr(CO)5PCl3] with carbonylcyclopentadienylmolybdenates and ‐ferrates. Complexes with cyclo‐P3 and cyclo‐P5 ligands (like 1), respectively, result.

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