Contents 1. Introduction 4236 2. The Stability of P 4 Phosphorus with Respect to Its Allotropes and Neutral Polyphosphorus Species 4237 2.1. Allotropic Modifications of Phosphorus 4237 2.2. Stability of Neutral Polyphosphorus Species 4238 3. General Trends of P 4 Phosphorus Activation 4239 3.1. General Remarks 4239 3.2. Degradation of P 4 Phosphorus by Nucleophiles under Maintenance of P n Structural Moieties 4241 4. Activation and Degradation of P 4 Phosphorus by Main Group Elements and Compounds 4243 4.1. Activation of P 4 Phosphorus by Group 1 and 2 Elements and Compounds 4243 4.2. Activation of P 4 Phosphorus by Group 13 Element Compounds 4245 4.3. Activation and Degradation of P 4 Phosphorus by Group 14 Element Compounds 4246 4.4. Activation and Degradation of P 4 Phosphorus by Group 15 Element Compounds 4250 4.5. Activation and Degradation of P 4 Phosphorus by Group 16 and 17 Element Compounds 4251 5. Abbreviations 4252 6. Acknowledgments 4253 7. References 4253

The reaction of [Cp*Fe(eta5-P5)] with Cu(I)Cl in solvent mixtures of CH2Cl2/CH3CN leads to the formation of entirely inorganic fullerene-like molecules of the formula [[Cp*Fe(eta5:eta1:eta1:eta1:eta1:eta1-P5)]12[CuCl]10[Cu2Cl3]5[Cu(CH3CN)2]5] (1) possessing 90 inorganic core atoms. This compound represents a structural motif similar to that of C60: cyclo-P5 rings of [Cp*Fe(eta5-P5)] molecules are surrounded by six-membered P4Cu2 rings that result from the coordination of each of the phosphorus lone pairs to CuCl metal centers, which are further coordinated by P atoms of other cyclo-P5 rings. Thus, five- and six-membered rings alternate in a manner comparable to that observed in the fullerene molecules. The so-formed half shells are joined by [Cu2Cl3]- as well as by [Cu(CH3CN)2]+ units. The spherical body has an inside diameter of 1.25 nanometers and an outside diameter of 2.13 nanometers, which is about three times as large as that of C60.

Terminal metal phosphinidene complexes (L n M = PR) are, despite continued interest, far less developed than their isolobal metal imide and alkylidene counterparts.[1] Although the first L n M=PR complex was reported over a quarter of a century ago, [2] sterically demanding R-groups are required, as M=PR linkages are reactive and require kinetic stabilization.[3] Certainly, free phosphinidenes (PR) are usually very reactive owing to their triplet ground states and unsaturated valence shells.[4] Although stabilization of a triplet PR group by a triplet metal fragment to generate a formal M=P bond is an attractive strategy, unlike the well-known L n M=NH and L n M=CH 2 linkages, [5] there has never been a structurally authenticated report of a d-/f-block metal-stabilized terminal parent phosphinidene L n M = PH, [6,7] and studies of such species are limited to computational investigations. [8] This paucity is underscored by a triplet-singlet energy gap of 22 kcal mol À1 for free PH, [4b] which has only been observed transiently in the gas phase or low temperature matrices. [4a, 9]

Mild thermolysis of Lewis base stabilized phosphinoborane monomers R1R2P–BH2⋅NMe3 (R1,R2=H, Ph, or tBu/H) at room temperature to 100 °C provides a convenient new route to oligo- and polyphosphinoboranes [R1R2P-BH2]n. The polymerization appears to proceed via the addition/head-to-tail polymerization of short-lived free phosphinoborane monomers, R1R2P-BH2. This method offers access to high molar mass materials, as exemplified by poly(tert-butylphosphinoborane), that are currently inaccessible using other routes (e.g. catalytic dehydrocoupling).

To further our fundamental understanding of the nature and extent of covalency in uranium-ligand bonding, and the benefits that this may have for the design of new ligands for nuclear waste separation, there is burgeoning interest in the nature of uranium complexes with soft- and multiple-bond-donor ligands. Despite this, there have so far been no examples of structurally authenticated molecular uranium-arsenic bonds under ambient conditions. Here, we report molecular uranium(IV)-arsenic complexes featuring formal single, double and triple U-As bonding interactions. Compound formulations are supported by a range of characterization techniques, and theoretical calculations suggest the presence of polarized covalent one-, two- and threefold bonding interactions between uranium and arsenic in parent arsenide [U-AsH2], terminal arsinidene [U=AsH] and arsenido [U≡AsK2] complexes, respectively. These studies inform our understanding of the bonding of actinides with soft donor ligands and may be of use in future ligand design in this area.

Despite the burgeoning field of uranium-ligand multiple bonds, analogous complexes involving other actinides remain scarce. For thorium, under ambient conditions only a few multiple bonds to carbon, nitrogen, oxygen, sulfur, selenium and tellurium are reported, and no multiple bonds to phosphorus are known, reflecting a general paucity of synthetic methodologies and also problems associated with stabilising these linkages at the large thorium ion. Here we report structurally authenticated examples of a parent thorium(IV)–phosphanide (Th–PH2), a terminal thorium(IV)–phosphinidene (Th=PH), a parent dithorium(IV)–phosphinidiide (Th–P(H)-Th) and a discrete actinide–phosphido complex under ambient conditions (Th=P=Th). Although thorium is traditionally considered to have dominant 6d-orbital contributions to its bonding, contrasting to majority 5f-orbital character for uranium, computational analyses suggests that the bonding of thorium can be more nuanced, in terms of 5f- versus 6d-orbital composition and also significant involvement of the 7s-orbital and how this affects the balance of 5f- versus 6d-orbital bonding character.

The abstraction of the Lewis acid from [W(CO)(5)(PH(2)BH(2)NMe(3))] (1) by an excess of P(OMe(3))(3) leads to the quantitative formation of the first Lewis base stabilized monomeric parent compound of phosphanylborane [H(2)PBH(2)NMe(3)] 2. Density functional theory (DFT) calculations have shown a low energetic difference between the crystallographically determined antiperiplanar arrangement of the lone pair and the trimethylamine group relative to the P-B core and the synperiplanar conformation. Subsequent reactions with the main-group Lewis acid BH(3) as well as with an [Fe(CO)(4)] unit as a transition-metal Lewis acid led to the formation of [(BH(3))PH(2)BH(2)NMe(3)] (3), containing a central H(3)B-PH(2)-BH(2) unit, and [Fe(CO)(4)(PH(2)BH(2)NMe(3))] (4), respectively. In oxidation processes with O(2), Me(3)NO, elemental sulfur, and selenium, the boranylphosphine chalcogenides [H(2)P(Q)BH(2)NMe(3)] (Q = S 5 b; Se 5 c) as well as the novel boranyl phosphonic acid [(HO)(2)P(O)BH(2)NMe(3)] (6 a) are formed. All products have been characterized by spectroscopic as well as by single-crystal X-ray structure analysis.

The structures of the parent compounds of phosphanyl- and arsanylboranes, H(2)BPH(2) and H(2)BAsH(2), were calculated by DFT-B3LYP methods. Such compounds have not previously been obtained preparatively. By applying the concept of Lewis acid/base stabilisation, [(CO)(5)W(H(2)EBH(2).NMe(3))] (E=P (3), As (4)) derivatives have been synthesised by the metathesis reactions between Li[(CO)(5)WEH(2)] and ClH(2)BNMe(3) (E=P, As). Comprehensive thermodynamic studies on these systems verify the high stability of the Lewis acid/base stabilised complexes. Unexpected based on the thermodynamic calculations, UV radiation of the phosphanylborane 3 leads to the dinuclear phosphanido-bridged complex [(CO)(8)W(2)(mu-PHBH(2).NMe(3))(2)] (5) by H(2) and CO elimination.