The Poisson's ratio of a material characterizes its response to uniaxial strain. Materials normally possess a positive Poisson's ratio - they contract laterally when stretched, and expand laterally when compressed. A negative Poisson's ratio is theoretically permissible but has not, with few exceptions of man-made bulk structures, been experimentally observed in any natural materials. Here, we show that the negative Poisson's ratio exists in the low-dimensional natural material black phosphorus and that our experimental observations are consistent with first-principles simulations. Through applying uniaxial strain along armchair direction, we have succeeded in demonstrating a cross-plane interlayer negative Poisson's ratio on black phosphorus for the first time. Meanwhile, our results support the existence of a cross-plane intralayer negative Poisson's ratio in the constituent phosphorene layers under uniaxial deformation along the zigzag axis, which is in line with a previous theoretical prediction. The phenomenon originates from the puckered structure of its in-plane lattice, together with coupled hinge-like bonding configurations.

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One of the two isomeric forms of the complex Re2Br4(a-dppm)2(CO) (lb), both of which are formed by the reaction of the triply bonded complex Re2Br4(u-dppm)2 with CO, is shown to have the same A-frame-like stmcture as that characterized previously for RejCLi/t-dppmHCO) (la) by X-ray crystallography. Crystal data for Re2-Br4(M-dppm)2(CO)'CH2Cl2 (lb) at 293 K: monoclinic space group Cc (No. 9) with a = 19.584(5) k,b = 17.109-(2) Á, c = 17.649(6) Á, ß = 105.92(2)°, V = 5686(5) Á3, Z = 4. The structure was refined to R = 0.055 (Rw = 0.073) for 4891 data with I > 3.0 (7). The CO ligand is terminally bound, and the Re-Re distance of 2.336-(1) Á accords with the retention of a Re=Re bond. The second isomeric form of Re2X4(w-dppm)2(CO) has been identified as having an open unsymmetrical structure X3Re(w-dppm)2ReX(CO). The reaction of lb with xylyl and feri-butyl isocyanides produces bioctahedral complexes of the type (RNC)Br2Re(a-dppm)2ReBr2(CO) (R = xylyl (3a), -Bu (3b)), in which two Re-Br bonds are collinear with the Re=Re bond, and the terminally bound RNC and CO ligands are anti to one another on separate Re centers. This structure has been established by a single-crystal X-ray structure determination of 3b. Crystal data for Re2Br4(M-dppm)2(CO)(CN-f-Bu)iCH2-CL^.SCeHe (3b) at 203 K: monoclinic space group P2\ln (No. 14) with a = 12.892(2) Á, b = 19.478(4) Á, c = 27.231(4) ß = 93.623(13)°, V = 6824(4) Á3, Z = 4. The structure was refined to R = 0.066 (Rw = 0.084) for 5166 data with I > 3.0 ( ). The Re-Re distance is 2.381(1) Á. The mixed carbonyl-isocyanide complexes 3a and 3b react with TIO3SCF3 in dichloromethane to give the triply-bonded compounds [RejBnfudppm)2(C0)(CNR)]03SCF3 (R = xylyl (4a), f-Bu (4b)), which have an unsymmetrical structure with the two metal centers possessing different coordination numbers and the CO and RNC ligands trans to one another. This conclusion is based upon a single-crystal X-ray structure determination of 4a. Crystal data for [ReiBrjfMdppm)2(C0)(CNxyl)]03SCF3 (4a) at 293 K: monoclinic space group P2j/c (No. 14) with a = 14.182(3) k,b = 15.749(4) Á, c = 31.580(5) Á, ß = 98.58(2)°, V = 6974(5) Á3, Z = 4. The structure was refined to R = 0.066 (Rw = 0.088) for 5333 data with I > 3.0 ( ). A comparison is made between these results and those obtained previously for the related dirhenium chloro complexes which contain CO and RNC ligands.* Reactions of the Dirhenium(II) Complexes Re2X4(dppm)z (X = Cl, Br; dppm = Ph^PClfiPPlu) with Isocyanides. 7. For part 6, see ref 24.