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Textbooks of inorganic chemistry describe the formation of adducts by coordination of an electron donor to an electron acceptor, often using the amine-boranes, X,N + BY,, as examples. In the Lewis (electron dot) formulas of the compounds, the dative bond in H,N -+ BH, and the covalent bond in H,C-CH, are both represented by a shared electron pair. In the simple molecular orbital or valence bond models the wave functions of both electron pairs would be constructed in the same manner from the appropriate sp3 type atomic orbitals on the bonded atoms: the difference between the covalent and the dative bond becomes apparent only after the orbital coefficients have been analyzed. This may be the reason why many structural chemists seem reluctant to distinguish between the two types of bonds. The object of this article is to remind the reader that the physicochemical properties of covalent and dative bonds may be-and often are-quite different, and to show that a distinction between the two provides a basis for understanding the structures of a wide range of main group metal compounds.

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The molecular structures of ferrocene in the eclipsed (equilibrium) and staggered (saddle-point) conformations have been determined by full geometry optimizations at the levels of second-order Møller-Plesset (MP2) theory, coupled-cluster singles-and-doubles (CCSD) theory, and CCSD theory with a perturbative triples correction [CCSD(T)] in a TZV2P+f basis set. Existing experimental results are reviewed. The agreement between the CCSD(T) results and experiment is in all cases excellent; the calculated structure parameters and the barrier to internal rotation of the ligand rings differ from the most accurate experimental values by less than two estimated standard deviations. The CCSD(T) calculations for single-configuration-dominated transition metal complexes such as ferrocene thus appear to have an accuracy comparable to that observed for molecules containing only first- and second-row atoms, and to be of a quality similar to that obtained experimentally. A comparison with previous DFT results indicates that the B3LYP model gives overall the best DFT results, with a deviation of around 2 pm for the metal-carbon distance and smaller errors for the cyclopentadienyl rings.

The accidental and independent discovery of dicyclopentadienyliron, now better known as ferrocene, was reported by two research groups in 1951 and 1952.1 This was the first pure hydrocarbon derivative of iron to be prepared, and its exceptional stability caused considerable excitement. Ferrocene is not only unaffected by air, moisture, and heat up to 470 °C, it can be boiled in concentrated hydrochloric acid or 10% caustic soda without decomposition. Even somebody Arne Haaland received his Ph.D. with W. H. Eberhardt at Georgia Institute of Technology in 1962. Upon his return to Norway he joined the electron-diffraction group under the leadership of Otto Bastlansen at the University of Oslo, where he Is now senior lecturer. His research interest centers on structure and bonding In organometallic compounds. He has been active in Amnesty International and spent the years [1975][1976][1977] as visiting professor at the

Bis[bis(trimethylsilyl)methyl]germanium, GeR, [ R = CH (SiMe,),], is conveniently prepared from GeCl,(diox) (diox = 1,4-dioxane) (for which an improved synthesis, from GeCI, and SnHBu",, is reported) and 2MgCI( R)(OEt,), MgR,(diox),.,, or MgR,(OEt,) in OEt,. The corresponding tin(ii) alkyl is accessible from SnCI, and successively 2LiR (to yield SnCI,R,) and Li,(cot) (cot = cyclooctatetraene) in OEt,. Gas-phase electron diffraction (g.e.d.) patterns, recorded with nozzle temperatures of 155 "C for GeR, and 1 2 0 "C for SnR,, show that the gas consists of V-shaped monomers. Least-squares refinements of models of C, symmetry yielded the bond distances Ge-C 203.8(15) and Sn-C 222(2) p m and the angles CGeC 107(2) and CSnC 97(2)". In GeR, the -CH(SiMe,), ligands are oriented so that the HCIMCIH moiety (Cl = inner, or methine, C) has a nearly planar syn,syn conformation but in SnR, the dihedral angles C'MC'H are ca. 15". Ab initio molecular-orbital calculations with a better than DZ (double zeta) basis were carried out on the model compounds MH, and GeMe,, and yielded the optimal bond distances Ge-H 158, Ge-C 202, and Sn-H 177 pm, and valence angles HGeH 93, CGeC 97, and HSnH 93". Correlation of experimental and calculated structures shows that M'l-H and M"-C bond distances are significantly larger (by 4-1 0 pm) than their MIV-H and MIV-C counterparts, and valence angles a t M increase in the series GeH, = SnH, < GeMe, = SnMe, < GeR, < SnR,. Both the long bond distances and the less-thantetrahedral valence angles are rationalised by assuming the metal lone pair to occupy atomic orbitals of predominant s character and the M-H and M-C o-bonding orbitals to be formed from metal atomic orbitals of predominant p character.

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The structure, energetics, and electron density in the inclusion complex of He in adamantane, C10H16, have been studied by density functional theory calculations at the B3LYP6-311++G(2p,2d) level. Topological analysis of the electron density shows that the He atom is connected to the four tertiary tC atoms in the cage by atomic interaction lines with (3,-1) critical points. The calculated dissociation energy of the complex He@adamantane(g)=adamantane(g) + He(g) of DeltaE=-645 kJ mol(-1) nevertheless shows that the He-tC interactions are antibonding.