The nature of the interaction between chloromethanes CH 4-n Cl n and Pt(II) complexes has been studied by highpressure X-ray diffraction and infrared spectroscopy in combination with DFT calculations. In case of electron rich complexes such as d 8 -Pt(btz-N,N′)(phenyl)L with L = phenyl, Cl, Br and btz = 2,2′-Bi-5,6-dihydro-4H-1,3-thiazine stable chloroform adducts with bridging hydrogen atoms in the η 1 (C-H)Pt moieties were isolated which display highly activated C-H bonds. This activa-The activation of carbon-hydrogen bonds is usually hampered by their rather apolar covalent character and large bond dissociation energies. For example the C-H bond dissociation enthalpies in simple alkanes such as methane [DH 298 = 439.28(13) kJ mol -1 ] are virtually as large as in the H 2 molecule [DH 298 = 435.998(13) kJ mol -1 ] displaying the prototype of a strong covalent bond. [1] As a consequence, alkanes are neither good electron donors nor good acceptors since the σ(C-H) bonding orbital is low in energy while the antibonding σ*(C-H) orbital is high lying. Hence, C-H bonds are generally considered to be chemically rather inert and their selective activation remains a challenge in organometallic chemistry. [2][3][4] This obstacle can be overcome by metal-assisted C-H bond activation in cases where an alkane ligand coordinates either end-on (η 1 ) or side-on (η 2 ) to a metal-ligand fragment ML n (Scheme 1). [5,6] In case of electron-rich late transition metal complexes two bonding scenarios with short M···H br -C contacts are usually observed for methane and halomethane d 8 -Pt complexes, where H br denotes a bridging hydrogen atom. These are illustrated in case of the theoretical model systems (CH 3 ) 2 Pt(NH 3 )(CH 4 ) 1a and (CH 3 ) 2 Pt(NH 3 ) 2 ·(CHCl 3 ) 1b in Scheme 1. We note, that all DFT calculations were performed with ADF using the BP86 functional, the ZORA for the descrip- [a]

We outline in this combined experimental and theoretical NMR study that sign and magnitude of J(Si,H) coupling constants provide reliable indicators to evaluate the extent of the oxidative addition of Si-H bonds in hydrosilane complexes. In combination with experimental electron density studies and MO analyses a simple structure-property relationship emerges: positive J(Si,H) coupling constants are observed in cases where M → L π-back-donation (M = transition metal; L = hydrosilane ligand) dominates. The corresponding complexes are located close to the terminus of the respective oxidative addition trajectory. In contrast negative J(Si,H) values signal the predominance of significant covalent Si-H interactions and the according complexes reside at an earlier stage of the oxidative addition reaction pathway. Hence, in nonclassical hydrosilane complexes such as CpTi(PMe)(HSiMeCl) (with n = 1-3) the sign of J(Si,H) changes from minus to plus with increasing number of chloro substituents n and maps the rising degree of oxidative addition. Accordingly, the sign and magnitude of J(Si,H) coupling constants can be employed to identify and characterize nonclassical hydrosilane species also in solution. These NMR studies might therefore help to reveal the salient control parameters of the Si-H bond activation process in transition-metal hydrosilane complexes which represent key intermediates for numerous metal-catalyzed Si-H bond activation processes. Furthermore, experimental high-resolution and high-pressure X-ray diffraction studies were undertaken to explore the close relationship between the topology of the electron density displayed by the η(Si-H)M units and their respective J(Si,H) couplings.

The vibrational response of the activated C–D bond in the chloroform complex [Pt(C 6 H 5 ) 2 (btz- N , N ′)·CDCl 3 , where btz = 2,2′-bi-5,6-dihydro-4 H -1,3-thiazine] is studied by linear and nonlinear two-dimensional infrared (2D-IR) spectroscopy. The change of the C–D stretching vibration of metal-coordinated CDCl 3 relative to the free solvent molecule serves as a measure of the non-classical Pt···D–C interaction strength. The stretching absorption band of the activated C–D bond displays a red shift of 119 cm –1 relative to uncoordinated CDCl 3 , a strong broadening, and an 8-fold enhancement of spectrally integrated absorption. The infrared (IR) absorption and 2D-IR line shapes are governed by spectral diffusion on 200 fs and 2 ps time scales, induced by the fluctuating solvent CDCl 3 . The enhanced vibrational absorption and coupling to solvent forces are assigned to the enhanced electric polarizability of the activated C–D bond. Density functional theory calculations show a significant increase of C–D bond polarizability of CDCl 3 upon coordination to the 16 valence electron Pt(II) complex.