Ring borylation of [Me4C2(eta5-C5H4)2CrCO] by B(C6F5)3 affords the zwitterionic complex {Me4(eta5-C5H4)(eta5-C4H3B(C6F5)3)}CrH(CO) (1), the first structurally characterized bent-metallocene complex of Cr(4+). This species decomposes thermally to the zwitterionic species {Me4(eta5-C5H4)(eta5-C4H3B(C6F5)3)}Cr (2) and the ionic species [Me4C2(eta5-C5H4)2CrCO][HB(C6F5)3] (3). The molecular structure of 2 is also described.

ansa-Calcocene compounds are effective reagents for the synthesis of ansa-chromocene complexes from CrCl 2 in the presence of a trapping ligand such as carbon monoxide or an isonitrile. A variety of ansa-chromocene carbonyl and tert-butyl isocyanide complexes have been prepared in this manner in high yields. The X-ray crystal structure of one of these complexes,CrCO, is described. Electrochemical studies on these complexes show that the isonitrile derivatives are more easily oxidized than the carbonyl derivatives. Preliminary examination of the reactivity of these complexes indicates that the nature of the substitution along the ethanediyl ansa-bridge influences the relative stabilities of the carbonyl complexes to oxidation in air, the ease with which the carbonyl ligands undergo substitution with tert-butyl isocyanide, and the relative sensitivities of the tert-butyl isocyanide adducts to photodecomposition. The ansa-bridge substitution also appears to influence the ability of the complexes to undergo structural changes, such as ring slippage, as revealed in their cyclic voltammograms.

Protonation of the metal occurs upon reaction of Me4C2(C5H4)2CrCO with [H(Et2O)2][B(3,5-(CF3)2C6H3)4] at −70 °C in thf-d 8. The diamagnetic Cr(IV) hydride intermediate was characterized by 1H NMR spectroscopy. Its decomposition to a paramagnetic product mixture upon warming to room temperature is accompanied by the evolution of 0.5 equiv of gas that is 54% H2 and 46% CO. Crystals of the borate salt of the 17e- ansa-chromocenium carbonyl cation [Me4C2(C2H4)2CrCO]+ were isolated from the product mixture for a molecular structure determination.

Addition of isonicotinic acid NC(5)H(4)CO(2)H (or isonicH) to [Pt(dppf)(MeCN)(2)](2+)2OTf(-)(dppf = 1,1'-bis(diphenylphosphino)ferrocene, OTf = triflate) affords a mixture of the homometallic molecular square [Pt(4)(dppf)(4)(mu-O(2)CC(5)H(4)N)(4)](4+)4OTf(-), 1 and its precursor intermediate [Pt(dppf)(eta(1)-NC(5)H(4)CO(2)H)(2)](2+)2OTf(-), 2. The latter captures [Pd(dppf)(MeCN)(2)](2+)2OTf(-) to give a heterometallic square, [Pt(2)Pd(2)(dppf)(4)(mu-O(2)CC(5)H(4)N)(4)](4+)4OTf(-), 3. Slight skeletal modification of the ligand leads to different assemblies. This is illustrated by the addition of NC(5)H(4)CH(2)CO(2)H.HCl to [Pt(dppf)(MeCN)(2)](2+)2OTf(-) to give [PtCl(dppf)(NC(5)H(4)CH(2)CO(2)H)](+)OTf(-), 4, which reacts with another equivalent of AgOTf to yield the dibridged complex [Pt(2)(dppf)(2)(mu-NC(5)H(4)CH(2)CO(2))(2)](2+)2OTf(-), 5. All complexes, with the exception of , have been structurally characterized by single-crystal X-ray crystallography. Complexes 2 and 4 are potential precursors to further molecular topologies.

A variety of 18e ansa-chromocene isocyanide complexes were prepared via ligand substitution on the carbonyl complexes. The relative π-accepting character of the different isocyanide ligands was evaluated from the structural, spectroscopic, and electrochemical properties of the complexes. One-electron chemical oxidation of the complexes generates low-spin, 17e ansa-chromocene isocyanide cations, the structural and spectroscopic properties of which were compared with their neutral precursors. Density functional theory (DFT) calculations were performed on model complexes in order to elucidate the nature of the bonding between the chromium and the isocyanide ligands and to explain the effects of one-electron oxidation of the complexes.