From the reactions between M2(OCH2tBu)6 (2 equiv) and NaP(SiMe3)2 (1 equiv) in benzene and 18-crown-6 the phosphinidine clusters [M4(μ3-PSiMe3)(OCH2tBu)11]-, compounds 1 (M = Mo) and 2 (M = W) are formed and are reactive intermediates in the formation of [M4(μ3-P)(OCH2tBu)10]- cluster anions, compounds 3 (M = Mo) and 4 (M = W), respectively, which have been isolated and characterized as their Na(18-crown-6)+ salts. Crystals of Na(18-crown-6)+[Mo4(μ3-P)(OCH2tBu)10]-·C6H6 were examined by single crystal X-ray crystallography at −110 °C in the space group Pn with a = 13.000(2) Å, b = 17.501(3) Å, c = 18.703(2) Å, and β = 93.11(1)° with Z = 2 and at −173 °C in the space group Cc with a = 22.118(4) Å, b = 17.480(3) Å, c = 23.250(4) Å, and β = 110.82(1)° with Z = 4. At the lower temperature, there is higher symmetry and no disorder of the 18-crown-6 molecule. The cluster anion has a Mo4 butterfly with a μ3-P ligand and virtual mirror symmetry. One wing-tip Mo atom is five-coordinate and bonded to the μ3-P atom and shows longer Mo-to-Mo distances (2.68(1) and 2.71(1) Å) relative to its counterpart (2.48(1) and 2.50(1) Å); the backbone Mo−Mo distance is 2.53(1) Å. The cluster anion in 3 but not 4 is also accessible from the reaction between Mo2(OCH2tBu)6 (2 equiv), PH3, and KOCH2tBu (1 equiv) in THF/18-crown-6 (THF = tetrahydrofuran). A similar approach involving AsH3, Mo2(OCH2tBu)6, and KOCH2tBu gave the μ3-arsenide cluster complex K(18-crown-6)+[Mo4(μ3-As)(OCH2tBu)10]-, compound 5, which by NMR spectroscopy is analogous in structure to 3. The reaction between Mo2(OCH2tBu)6 (2 equiv) and KOH in THF/18-crown-6 gave K(18-crown-6)2+[Mo4(μ3-O)(OCH2tBu)11]-, compound 6, which is proposed to be isostructural with the phosphinidine complexes 1 and 2 on the basis of NMR data. Crystal data for K(η6-C12H24O6)(η2-C12H24O6)+[Mo4(μ3-O)(OCH2tBu)11]- at −165 °C: a = 17.595(2) Å, b = 28.044(3) Å, c = 20.443(2) Å, β = 97.61(1)° with Z = 4, and space group P21/n. The cluster anion in 6 contains a 12-electron Mo4 butterfly with 12 M−M bonding electrons, as in 3, but the shortest Mo−Mo distance 2.42(1) Å involves the backbone metal atoms. The Mo−Mo distances involving the oxo-capped triangle (2.61(1) and 2.62(1) Å) are notably longer than those in the other triangle (2.48(1) Å).