Analysis of accurate experimental and theoretical structure factors of diamond and silicon reveals that the contraction of the core shell due to covalent bond formation causes significant perturbations of the total charge density that cannot be ignored in precise charge density studies. We outline that the nature and origin of core contraction/expansion and core polarization phenomena can be analyzed by experimental studies employing an extended Hansen-Coppens multipolar model. Omission or insufficient treatment of these subatomic charge density phenomena might yield erroneous thermal displacement parameters and high residual densities in multipolar refinements. Our detailed studies therefore suggest that the refinement of contraction/expansion and population parameters of all atomic shells is essential to the precise reconstruction of electron density distributions by a multipolar model. Furthermore, our results imply that also the polarization of the inner shells needs to be adopted, especially in cases where second row or even heavier elements are involved in covalent bonding. These theoretical studies are supported by direct multipolar refinements of X-ray powder diffraction data of diamond obtained from a third-generation synchrotron-radiation source (SPring-8, BL02B2).

Postsynthetic metal ion exchange in a benzotriazolate-based MFU-4l(arge) framework leads to a Co(II)-containing framework with open metal sites showing reversible gas-phase oxidation properties.

The synthesis of a series of heteroleptic ate-complex-free rare-earth(III) diamide complexes by alkane and amine elimination reactions, starting from Ln(CH 2 SiMe 3 ) 3 (THF) 2 and Ln-(NiPr 2 ) 3 (THF) x (x ) 1, Ln ) Sc, Lu; x ) 2, Ln ) Y), respectively, is described. 2,6-Bis(((2,6diisopropylphenyl)amino)methyl)pyridine (H 2 BDPPpyr) formed monomeric complexes of the types (BDPPpyr)Ln(CH 2 SiMe 3 )(THF) x (x ) 1, Ln ) Sc, Lu; x ) 2, Ln ) Y) and (BDPPpyr)-Ln(NiPr 2 )(THF) (Ln ) Sc, Lu), which display enhanced stability for the smaller metal center scandium, for diisopropylamide coordination, and in donor solvents such as THF. Conversion of the silylalkyl complexes into their amide derivatives via secondary alkane elimination, i.e., reaction with HNEt 2 and HN(SiHMe 2 ) 2 , increased the complex stability. The mono-THF adduct complexes (BDPPpyr)Sc(L)(THF) show a nonfluxional structure in solution, which contrasts the dynamic behavior of the corresponding bis-THF adduct complexes of the larger elements lutetium and yttrium. Sterically less encumbered 2,6-bis((mesitylamino)methyl)pyridine (H 2 BMespyr) gave the less stable complexes (BMespyr)Ln(CH 2 SiMe 3 )(THF) x (x ) 1, Ln ) Sc; x ) 2, Ln ) Lu; the Y derivative could not be isolated). Again, subsequent silylalkyl/silylamide ligand exchange gave the complexes (BMespyr)Ln[N(SiHMe 2 ) 2 ](THF), exhibiting considerably increased stability. The complexes (BDPPoxyl)Ln(CH 2 SiMe 3 )(THF) (Ln ) Sc, Lu, Y), derived from a nonfunctionalized diamide ligand (H 2 BDPPoxyl ) 1,2-bis-(((2,6-diisopropylphenyl)amino)methyl)benzene), were isolated in high yields. The 4-and 5-coordinate complexes (BDPPoxyl)Sc(CH 2 SiMe 3 )(THF) and (BDPPpyr)Sc(CH 2 SiMe 3 )(THF), respectively, were also characterized by X-ray diffraction structure determinations. All of the 5-coordinate scandium complexes derived from the BDPPpyr ligand effectively polymerize methyl methacrylate in a living manner (M w /M n < 1.5), affording mainly atactic polymer at ambient temperature.

The nature of metal silane σ-bond interaction has been investigated in several key systems by a range of experimental and computational techniques. The structure of [Cp′Mn(CO) 2 (η 2 -HSiHPh 2 )] 1 has been determined by single crystal neutron diffraction, and the geometry at the Si atom is shown to approximate a trigonal bipyramid; salient bond distances and angles are Mn-H(1) 1.575(14), Si-H(1) 1.806(14), Si-H(2) 1.501(13) Å, and H(1)-Si-H(2) 148.5(8)°. This complex is similar to [Cp′Mn(CO) 2 (η 2 -HSiFPh 2 )] 2, whose structure and bonding characteristics have recently been determined by charge density studies based on high-resolution X-ray and neutron diffraction data. The geometry at the Si atom in these σ-bond complexes is compared with that in other systems containing hypercoordinate silicon. The Mn-H distances for 1 and 2 in solution have been estimated using NMR T 1 relaxation measurements, giving a value of 1.56(3) Å in each case, in excellent agreement with the distances deduced from neutron diffraction. Density functional theory calculations have been employed to explore the bonding in the Mn-H-Si unit in 1 and 2 and in the related system [Cp′Mn(CO) 2 (η 2 -HSiCl 3 )] 3. These studies support the idea that the oxidative addition of a silane ligand to a transition metal center may be described as an asymmetric process in which the Mn-H bond is formed at an early stage, while both the establishment of the Mn-Si bond and also the activation of the η 2 -coordinated Si-H moiety are controlled by the extent of Mn f σ*(X-Si-H) back-donation, which increases with increasing electron-withdrawing character of the X substituent trans to the metal-coordinated Si-H bond. This delocalized molecular orbital (MO) approach is complemented and supported by combined experimental and theoretical charge density studies: the source function S(r,Ω), which provides a measure of the relative importance of each atom's contribution to the density at a specific reference point r, clearly shows that all three atoms of the Mn(η 2 -SiH) moiety contribute to a very similar extent to the density at the Mn-Si bond critical point, in pleasing agreement with the MO model. Hence, we advance a consistent and unifying concept which accounts for the degree of Si-H activation in these silane σ-bond complexes.