Experimental and theoretical charge density studies and molecular orbital analyses suggest that the complexes [Cp2Ti(PMe3)SiH2Ph2] (1) and [Cp2Ti(PMe3)SiHCl3] (2) display virtually the same electronic structures. No evidence for a significant interligand hypervalent interaction could be identified for 2. A bonding concept for transition-metal hydrosilane complexes aims to identify the true key parameters for a selective activation of the individual M-Si and Si-H bonds.

In general, C À H bonds can be considered chemically inert as a result of their strength, nonpolar nature, and low polarizability. Since the pioneering work of La Placa and Ibers in 1965, who reported the close approach of a CÀH bond to a transition-metal center, there have been many attempts to trace the microscopic control parameters of such C À H activation processes by metal atoms in general.[1] In particular, complexes containing side-on-coordinated (h 2 -CH) moieties next to a transition metal are the focus of intensive research as they allow the systematic study of the CÀH activation phenomenon in molecules and solids in their electronic ground states. Furthermore, M···H À C interactions (M = transition metal) play a key role in the performance of several industrially relevant catalytic processes, such as olefin polymerization.[2]In the course of a systematic analysis of such M···HÀC interactions, Brookhart and Green coined the expression agostic interactions to "discuss the various manifestations of covalent interactions between C À H groups and transitionmetal centers in organometallic compounds". [3a,b] In case of d 0 early-transition-metal alkyl or amido complexes, the strength of agostic interactions is mainly controlled by 1) the local Lewis acidity of the metal center, 2) the extent of negative hyperconjugative delocalization of the M À C/M À N bonding electrons, and 3) to a smaller degree by s(M ! H À C) donation.[3c, 4] For agostic late-transition-metal complexes, however, the control parameters are less clear. We therefore synthesized a variety of new Spencer-type [5] nickel alkyl cations 2 b-d by protonation of the corresponding olefin complexes 1 b-d to study the nature of their pronounced agostic interactions by combined experimental and theoretical charge density studies (Scheme 1).Re-examination of the classic Spencer-type complex [EtNi(dtbpe)]showed a fast rotation of the b-agostic methyl moiety in solution[6] and a systematic crystallographic disorder in the solid state, thus preventing a detailed investigation of the bonding properties of this agostic textbook example by experimental charge-density studies. We therefore replaced the ethylene moiety in 1 a by the sterically more demanding norbornyl (nbe) and dicyclopentadienyl (DCp) ligands. Protonation of 1 b-d yielded the agostic complexes 2 b-d, which all have a significantly reduced fluxional behavior in solution. Furthermore, single crystals of excellent quality could be obtained for 2 b-d, which even allowed an experimental charge-density analysis of 2 c.

In this work we report on the syntheses and properties of several new Ni complexes featuring the chelating bisguanidines bis(tetramethylguanidino)benzene (btmgb), bis(tetramethylguanidino)naphthalene (btmgn), and bis(tetramethylguanidino)biphenyl (btmgbp) as ligands. All complexes were structurally characterized by single-crystal X-ray diffraction and quantum chemical calculations. A detailed inspection of the magnetic susceptibility of [(btmgb)NiX(2)] and [(btmgbp)NiX(2)] (X=Cl, Br) revealed a linear temperature dependence of chi(-1)(T) above 50 K, which was in agreement with a Curie-Weiss-type behavior and a triplet ground state. Below approximately 25 K, however, magnetic susceptibility studies of the paramagnetic d(8) Ni complexes revealed the presence of a significant zero-field splitting (ZFS) that results from spin-orbit mixing of excited states into the triplet ground state. The electronic consequences that might arise from the mixing of states as well as from a possible non-innocent behavior of the ligand have been explored by an experimental charge density study of [(btmgb)NiCl(2)] at low temperatures (7 K). Here, the presence of ZFS was identified as one potential reason for the flat angle-spherical Cl-Ni-Cl deformation potential and the distinct differences between the angle-spherical X-Ni-X valence angles observed by experiment and predicted by DFT. An analysis of the topology of the experimentally and theoretically derived electron-density distributions of [(btmgb)NiCl(2)] confirmed the strong donor character of the bisguanidine ligand but clearly ruled out any significant non-innocent ligand (NIL) behavior. Hence, [(btmgb)NiCl(2)] provides an experimental reference system to study the mixing of certain excited states into the ground state unbiased from any competing NIL behavior.

The electronic structures of the isotypic carbides Sc3TC4 (see picture; T=Fe, Co, Ni) are investigated by theoretical and experimental charge‐density studies. Even tiny differences in the electronic band structure of these solids are reflected in the properties of the Laplacian of the experimental electron density. Only the cobalt carbide is superconducting below 4.5 K and displays a structural phase transition around 70 K.

In general, it is assumed that the reaction between silanes and late transition metal fragments yields silyl hydride species as oxidative addition products. However, the silane complex Ni(iPr 2 Im) 2 (SiHMePh 2 ) (iPr 2 Im = 1,3-diisopropylimidazolin-2-ylidene) (3a), might represent one of the rare systems where a stable η 2 -(Si-H)Ni intermediate of the oxidative addition process has been isolated. Indeed, 3a is characterized by an acute ЄSi-Ni-H angle of * Prof. Dr. U. Radius,