9-VERTEX POLYHEDRAL IRIDAMONOCARBABORANE CHEMISTRY - PRODUCTS OF THERMOLYSIS OF [(CO)(PPH(3))(2)IRCB7H8] AND EMERGING ALTERNATIVE CLUSTER-GEOMETRY PATTERNS

Thermolysis of [7-(CO)-7,7-(PPh(3))(2)-7.1-IrCB7H8] 1 at 250 degrees C resulted in isomerisation to give orange air-stable [7-(CO)-7-H-7,9-( PPh(3))(2)-7,1-IrCB7H7] 2 and its 6-(PPh(3)) isomer 3 as the major pro ducts. The compounds were characterised by NMR spectroscopy and single -crystal X-ray diffraction analysis on 1 and 3. The CH2Cl2 disolvate o f 1 is monoclinic, space group P2(1)ln, with a = 1394.33(10), b = 1276 .81(13), c = 2393.5(2) pm, beta = 94.163(9)(o), and Z = 4, and the str ucture was refined to R(R(g)) = 0.0351(0.0384) for 5991 reflections wi th I > 2.0 sigma(I). The CH2Cl2 monosolvate of 3 is triclinic, space g roup P1, with a = 1150.39(11), b = 1177.08(13), c = 1498.3(2) pm, alph a = 88.192(9), beta = 89.506(8), gamma = 80.720(9)(o), and Z = 2, and the structure was refined to R(R(g)) = 0.0299(0.0311) for 5864 reflect ions with I > 2.0 sigma(I). The nine-vertex {IrCB7} cluster structures of 1 and 3 have quadrilateral open faces (B-B ca. 220 pm) and therefo re 'isonido' geometries although of formal close constitution. The clu ster opening is briefly discussed in the context of a nine-vertex clos o-isonido-isocloso structural continuum, in the context of other conte mporaneously recognised structural continua in twelve-, eleven-, ten- and eight-vertex boron cluster chemistry, and thence in the context of emerging general structural patterns that interrelate the isocloso, i sonido and isoarachno cluster geometries.