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I 1 J The g tensors and linewidths vary slightly for different samples. A representative EPR spectrum gives g , = 2.26, g,, = 2.02 (solid-state at 77 K). H,,,0.32x0.04x0.04mm3, hexagonal, P6,22, a = b =12.1121(5), c =17.3157(10),&, V = 2199.9(2) A3, 2 = 6, psalcd = 1.800 g~m -~, Mo,,radiation ( ; . = 0.71073 A), p = 1.22 mm-'. Data were collected on a Siemens CCD Smart system in the range 4<20< 54". 11859 measured reflections, 1625 unique, 1276 with F,40(F,) were used to refine 98 parameters to R(wR) = 0.0479(0.0786), GOF = 1.185; F Z refinement in SHELXTL 5.0. A $J scan absorption correction gave min. and max. transmission factors of 0.79 and 0 91. The highest peak in the final difference map = 0.527 e k ' . Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publlcation no. CCDC-179.110. Copies of the data can be obtained free ofcharge on application to The Director,CCDC, 12Union Road,CambridgeCB21EZ,UK(fax: 1nt.code +(1223) 336-033; e-mail: teched@chemcrys.cam.ac uk). [14] a) The tight binding band structure calculations used an extended Huckel-type Hamiltonian: R. Hoffmann, J Chem. Phys. 1963, 39, 1397. b) A modified Wolfsberg-Helmholz formula was used to evaluate the nondiagonal H,, values: J. Ammeter, H.-B. Biirgi, J. Thibeault, R. Hoffmann, J Am. Chem. SOC. 1978,100,3686. c) The exponents and parameters used were taken from R. H. Summerville, R. Hoffmann, L Am. Chem. Soc. 1976, 98, 7240. I151 M.-H. Whangbo, R. Hoffmann, J Am. Chem. Soc. 1978, 100.6093, [16] Preliminary data support a localized system as a result of strong electron correlation effects in one dimension. Application of pressure (ca. 25 kbar) is expected to restore metallic character (room temperature conductivity = 2 Scm-'):

Apertureless scanning near-field optical microscopy (SNOM) in the reflection-back-to-the-fiber configuration is possible on rough surfaces of practical importance and reaches 15 nm lateral resolution. The feedback for constant distance mode is provided by shear-force atomic force microscopy. Any artifacts in the atomic force microscopy will translate into artificial optical responses. Very sharp tapered tips (radius ca. 15 nm, apex angle`10°) are required. Only these give the strongly enhanced reflectance in shear-force distance, do not break as easily as do more blunt tips, and can follow the topography in order to provide chemical SNOM contrast without topographic errors. In the absence of significant reflectance enhancement, if blunt or abrased or broken tips are used, not SNOM but artificial 'optical contrast' is recorded. Metal-coated tips are unsuitable: they become hot, give poor resolution and are the source of numerous additional artifacts that are summarized for comparison. The SNOM artifacts are subdivided into:* topographic errors-if chemical uniform topography does give some optical response; * lack of spatial correlation-if the optical signal does not reproduce the site of the emitter or of the chemical contrast; * tip imaging-very large features and more or less negative differentials therefrom; * contrast inversion-false sign of optical contrast; * split optical contrast-signal is split into two unequal parts with the same (false) sign of the contrast; * artificial stripes-instead of chemical/emissive contrast artificial stripes, that are not always 'interference fringes' or 'edge-contrast'.The artifacts can be easily recognized in published images from different techniques and modes of SNOM. Their analysis would be facilitated if full x/y/z data were published instead of two-dimensional images. We therefore disclose interactively analyzable VRML data in the EPOC version.

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