Nido complexes

C2B7H9)-, which could be rearranged to (1,2-C2B9Hu)Co(l, 10-C2B7H9) (66). No intermediate nido complexes were noted in this latter transformation, in contrast to the c-yclopentadienyl analog (see Section VII) although these species have been prepared (67) and appear to have both B—H—B and B—H—Co bridge bonds. 

The predictions of structures of transition metal-carbonyl complexes, using Wade s rules are often, but not always, accurate. For example, the clusters M4(CO)i2 (M = Co, Rh, Ir) have 60 valence electrons and are predicted to be nido complexes (14n + 4 valence electrons). A nido structure would correspond to a trigonal bipyramid... 

Note that MW of the samples synthesized using closo complexes, are noticeably lower as compared to the polymers obtained in the presence of either the exo-nido complexes or conventional radical initiators (such as AIBN and peroxides) where the molecular weights of polyacrylates range from several hundred thousands to several millions. Since mthenacarboranes with the closo stmcture are more promising for the synthesis of narrow-dispersed polymers, we studied the kinetics of MMA polymerization in the presence of the above mentioned catalysts and analyzed the molecular-weight characteristics of the polymers in more detail, using complex 4 as an example. 

Fairly close polydispersity values of polymer samples prepared in the presence of complexes 1 and 2 having different structures may be explained by the transformation of exo-nido complex 1 into its closo isomer 2, which is known to proceed in high yield at 80 °C 14). This transformation does not occur for the more sterically hindered C,C-dimethyl-substituted exo-nido complex 7, and, in this case, a polymer with a higher polydispersity index (M /Mn = 1.93) is formed. This finding indicates that the propagation step and the molecular-weight characteristics of PMMA are affected by both steric and stractural features of the rathenacarborane catalysts. 

An unusual property of aloeo—exo-nido complexes was their ability to exchange carbaborane units (Scheme 5]. The ease of displacement of the nido-carbaborane anions from eJoeo-Rh-complexes... 

R, R > CHI2 sequence was observed with exo-nido-complexes. 

This works well for all nido- and arachno-boranes but not for the c/o o-dianions, which are much less reactive. Reactions of B2H6 with NH3 are complex and, depending on the conditions, yield aminodiborane, H2B(/r-H)(/r-NH2)BH2, or the diammoniate of diborane, [BH2(NH3)2]-[BH4] (p. 165) at higher temperatures the benzene analogue borazine, (HNBH)3, results (see p. 210). 

Chromocene and nido-5,6-B2BgH,2 form the bimetallic chromium complex ... 

Several reports of carboranylated porphyrins have appeared for utilization in the BNCT of cancer.150 The examples include both nido- (126) (Fig. 73) and closo-carborane cluster-containing (124 and 125) (Fig. 73) materials. Metal-free nido-carboranylporphyrins were able to deliver a higher amount of boron to cells than the corresponding zinc complexes. 

Synthetic routes to metalloboranes are briefly summarized. Two specific aspects of the field are selected for more detailed comment (i) the occurrence of exopolyhedral cycloboronation of P-phenyl groups (ii) oxidative cluster closure reactions. These topics are illustrated by reference to reactions of closo-BioHio nido-B9H12 and arachno-BoHiU" with a variety of Ir1 complexes, and several novel metalloborane cluster geometries (as determined by X-ray diffraction analysis) are described, notably nido-TIr(BoHi )H (PPh3)2], i so-ni do-[ IrC(OH)B8H6(OMe) (CgH PPh )... 

T. Baker, DuPont Central Research In reference to your so-called iso-closo iridaborane complex, H(PPh3)2IrB9H9, we have recently published two papers (1,2) dealing with isoelec-tronic, isostructural ten-vertex ruthenacarborane complexes and have demonstrated that these structures are related to the common closo bicapped square antiprismatic structure by the removal of two electrons (i.e. 2N skeletal electrons for an N vertex polyhedron). Such complexes have been referred to as hyper-closo to imply that the electronic unsaturation is not primarily metal-based (as in, for example, nido-(PPh3)2RhC2B8-Hx 2 (3) or closo-(PPh3)ClRh(1,7-C,BQH,) (4), but is delocal-... 

Professor Greenwood has provided routes to metalloboranes which are derived from the noble metals Pt and Ir and the nido-B9H12 ", arachno-B9H1A, and closo-BioH102 ions. We wish to make note of some new reactions of Ir and Rh complexes with the arachno-B< H9 ion and compare these results with his observations of arachno-B9H14. ... 

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