A synthesis of the 2-phosphathioethynolate anion, PCS^–, under ambient conditions is reported. The coordination chemistry of PCO^–, PCS^– and their nitrogen-containing congeners is also explored. Photolysis of a solution of W(CO)₆ in the presence of PCO^– [or a simple ligand displacement reaction using W(CO)₅(MeCN)] affords [W(CO)₅(PCO)]^– (1). The cyanate and thiocyanate analogues, [W(CO)₅(NCO)]^– (2) and [W(CO)₅(NCS)]^– (3), are also synthesised using a similar methodology, allowing for an in-depth study of the bonding properties of this family of related ligands. Our studies reveal that, in the coordination sphere of tungsten(0), the PCO^– anion preferentially binds through the phosphorus atom in a strongly bent fashion, while NCO^– and NCS^– coordinate linearly through the nitrogen atom. Reactions between PCS^– and W(CO)₅(MeCN) similarly afford [W(CO)₅(PCS)]^–; however, due to the ambidentate nature of the anion, a mixture of both the phosphorus- and sulfur-bonded complexes (4a and 4b, respectively) is obtained. It was possible to establish that, as with PCO^–, the PCS^– ion also coordinates to the metal centre in a bent fashion.

The synthesis and isolation of the 2-arsaethynolate anion, AsCO⁻, and its subsequent reactivity towards heteroallenes is reported. Reactions with ketenes and carbodiimides afford four-membered anionic heterocycles in formal [2+2] cycloaddition reactions. By contrast, reaction with an isocyanate yielded a 1,4,2-diazaarsolidine-3,5-dionide anion and the unprecedented cluster anions As₁₀²⁻ and As₁₂⁴⁻. These preliminary reactivity studies hint at the enormous potential synthetic utility of this novel anion, which may be employed as an arsenide (As⁻) source.

The synthesis and isolation of the 2-arsaethynolate anion, AsCO⁻, and its subsequent reactivity towards heteroallenes is reported. Reactions with ketenes and carbodiimides afford four-membered anionic heterocycles in formal [2+2] cycloaddition reactions. By contrast, reaction with an isocyanate yielded a 1,4,2-diazaarsolidine-3,5-dionide anion and the unprecedented cluster anions As₁₀²⁻ and As₁₂⁴⁻. These preliminary reactivity studies hint at the enormous potential synthetic utility of this novel anion, which may be employed as an arsenide (As⁻) source.

We demonstrate that the synthesis of new N-functionalized phosphinecarboxamides is possible by reaction of primary and secondary amines with PCO⁻ in the presence of a proton source. These reactions proceed with varying degrees of success, and although primary amines generally afford the corresponding phosphinecarboxamides in good yields, secondary amines react more sluggishly and often give rise to significant decomposition of the 2-phosphaethynolate precursor. Of the new N-derivatized phosphinecarboxamides available, PH₂C(O)NHCy (Cy=cyclohexyl) can be obtained in sufficiently high yields to allow for the exploration of its Brønsted acidity. Thus, deprotonating PH₂C(O)NHCy with one equivalent of potassium bis(trimethylsilyl)amide (KHMDS) gave the new phosphide [PHC(O)NHCy]⁻. In contrast, deprotonation with half of an equivalent gives rise to [P{C(O)NHCy}₂]⁻ and PH₃. These phosphides can be employed to give new phosphines by reactions with electrophiles, thus demonstrating their enormous potential as chemical building blocks.

We report that the 2-phosphaethynolate anion (PCO⁻) reacts with propargylamines in the presence of a proton source to afford novel N-derivatized phosphinecarboxamides bearing alkyne functionalities. Deprotonation of these species gives rise to novel five- and six-membered anionic heterocycles resulting from intramolecular nucleophilic attack of the resulting phosphide at the alkyne functionality (via 5-exo-dig or 6-endo-dig cyclizations, respectively). The nature of the substituents on the phosphinecarboxamide can be used to influence the outcome of these reactions. This strategy represents a unique approach to phosphorus-containing heterocylic systems that are closely related to known organic molecules with interesting bio-active properties.

The reactivity of the 2-phosphaethynolate anion (PCO⁻) towards a cyclic trisilene (cSi₃(Tip)₄) is reported. The result is the net activation of the PC and SiSi multiple bonds of the precursors affording a heteroatomic bicyclo[1.1.1]pentan-2-one analogue ([P(CO)Si₃(Tip)₄]⁻; 1). This reaction can be interpreted as the formal addition of a phosphide and a carbonyl across the SiSi double bond. Photolytic decarbonylation of 1 results in the incorporation of the phosphide vertex into the cyclotrisilene scaffold, yielding a congener of the cyclobutene anion with considerable allylic character.

By following a similar procedure to that employed for the synthesis of [HP₇]^2– (1), the synthesis and characterization of the hydrogenheptaarsenide dianion [HAs₇]^2– (2) was achieved. Building on previous research that has shown that [HP₇]^2– can react with carbodiimides to yield amidine-functionalized cluster anions [P₇C(NHR)(NR)]^2–, we have found that the analogous hydroarsination reactions are also possible by reaction of 2 with RC=N=CR to yield [As₇C(NHR)(NR)]^2– [R = Dipp (3), iPr (4) and Cy (5); Dipp = 2,6-diisopropylphenyl]. Furthermore, we also demonstrate that such hydropnictination reactions can be extended to other heteroallenes such as isocyanates (O=C=NAd; Ad = adamantyl) to afford the amide-derivatized cluster anions [E₇C(CO)(NHAd)]^2– [E = P (6) and As (7)]. All new compounds were characterized by multinuclear NMR spectroscopy, elemental analyses and electrospray mass spectrometry. Clusters 2, 4, 6 and 7 were also characterized by single-crystal X-ray diffraction of [K(18-crown-6)]₂[2], [K(18-crown-6)]₂[4], [K(18-crown-6)]₂[6]·py (py = pyridine) and [K(18-crown-6)]₂[7]·py. Crystal structures confirming composition and connectivity were also obtained for anions 3 and 5.

Reaction of cyclooctatetraene (COT) iron(II) tricarbonyl, [Fe(cot)(CO)₃], with one equivalent of K₄Ge₉ in ethylenediamine (en) yielded the cluster anion [Ge₈Fe(CO)₃]³⁻ which was crystallographically-characterized as a [K(2,2,2-crypt)]⁺ salt in [K(2,2,2-crypt)]₃[Ge₈Fe(CO)₃]. The chemically-reduced organometallic species [Fe(η³-C₈H₈)(CO)₃]⁻ was also isolated as a side-product from this reaction as [K(2,2,2-crypt)][Fe(η³-C₈H₈)(CO)₃]. Both species were further characterized by EPR and IR spectroscopy and electrospray mass spectrometry. The [Ge₈Fe(CO)₃]³⁻ cluster anion represents an unprecedented functionalized germanium Zintl anion in which the nine-atom precursor cluster has lost a vertex, which has been replaced by a transition-metal moiety.