Metallacycles short atomic distances

Short nonbonded Si—Si distances have been observed in four membered metallacycles (Scheme 12b) with a Pt, Ir, W, or Nb atom [138-142] in place of one of the oxygen (nitrogen) atoms of 1,3-cyclodisilazanes (1,3-cyclodisilazanes) and in U-silylene-bridged dinuclear platinum complexes (Scheme 12c) [143, 144]. Electron donating occupied orbitals are expected to be on the platinum atoms like lone pair orbitals on the oxygen atoms in cyclodisiloxanes. 

Fig. 11 (a) A view of the unit cell of [Cp2Mo(dddt)][TCNQ]. (b) (c) Side view and top view of the dyad association with the short intermolecular S S distance (dotted line) involving the S atoms of the metallacycle... 

According to X-ray diffraction data, in the solid-state complex 87 adopts a six-membered metallacyclic structure, as the result of P-Pd-Cl-B bridging coordination. The interaction of the borane moiety with the chlorine atom is apparent from the short Cl-B distance (2.165(2) A) and the pyramidalization of the boron environment (EBa = 349.1°). The bridging coordination of the Pd-Cl bond by the PB 7b contrasts with the B-pendant coordination mode adopted by complex 72, deriving from the related ligand 7e featuring mesityl groups at boron (see Section IV.A). This illustrates the role of steric factors in the participation, or not, of the Lewis acid in the coordination assembly. 

This type of cycloaurated complexes display short transannular Au- Au interactions, as observed in the crystal structures of Au(bonding interaction between the metal centers. This attractive interaction is probably the reason for the distortion of the linear environment of the gold atoms. The metallacycle has a twist conformation, probably as a consequence of the steric requirements of the fairly bulky phenyl substituents. 

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