Transition metal-boron distances

Ligand Effects in Transition Metal-boron Distances. 172... 

In addition to the rj1, q2 and r 3 covalent transition metal tetrahydrobo-rates, another type of complexes exist where the interaction between M+ and BH4- is mainly electrostatic. These compounds which are quite common are defined as ionic tetrahydroborate complexes. They are characterised by the non-coordination of the tetrahydroborate and consequently by a long metal-boron distance. Several compounds of this class have been wrongly classified as monodentate covalent complexes. 

In Fig. 10, the average values of the metal-boron distances obtained from crystallographic data are depicted for each transition metal and each coordination mode. The results are given following the periodic order. In white, the results for the first transition metal row are presented, in grey, results for the second, and in black, results for the third. There are three bars for each transition metal on the left, the average of r)1 distances (plain), in the middle, the r 2 (squared), and at the right, the q3 mean values (vertical bordered). 

Strong boron-transition metal interaction interatomic distances are shorter by 5-10%, as compared to the sum of the metal radii. 

B—C Bond Distances in Transition Metal Clusters Containing Boron Atoms"... 

This heteroaromatic character of 1,2-azaborolyl rings qualifies them to form numerous transition metal complexes. In contrast to the cyclopentadienyl ligand there is still a remarkable disturbance caused by the boron and nitrogen atoms. This becomes visible if those transition metals are used for complexation that cannot reach the 18-electron configuration in normal sandwich complexes. As a consequence, electron-rich metals tend to increase the distance to the electron-rich nitrogen atoms, whereas electron-poor metals tend to increase the distance to the boron atom. Three examples illustrate these structural features of 1,2-azaborolyl rings. 

With respect to actinide intermetallics, the synthesis with boron is a powerful tool for tuning the localization of 5f states in AnT3 compounds, because boron occupies interstitial positions in the centre of the AuCu 3 unit cell. Even a stoichiometry corresponding to monoboride AnT3B can be prepared in this way. Since the T-B distance is smaller by about a factor of /3 than the An-B distance, the interaction with transition-metal electrons is more likely. This situation causes a dehybridization of 5f and transition-metal d-states and, together with the lattice expansion, a progressive localization. 

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