Complexes valence shell electron pair repulsion

The molecular geometry of a complex depends on the coordination number, which is the number of ligand atoms bonded to the metal. The most common coordination number is 6, and almost all metal complexes with coordination number 6 adopt octahedral geometry. This preferred geometry can be traced to the valence shell electron pair repulsion (VSEPR) model Introduced In Chapter 9. The ligands space themselves around the metal as far apart as possible, to minimize electron-electron repulsion. 

The molecular structures adopted by simple carbonyl complexes are generally compatible with predictions based on valence shell electron pair repulsion theory. Three representative examples from the first transition series are shown in Fig. 15.2. 

The geometric structure of the covalent binary halides, whether neutral or complexed ions, can be explained on the basis of the Nyhotm-Gillespie rules known as the Valence Shell Electron Pair Repulsion Model (VSEPR) theory the geometrical arrangements of the bonds around an atom in a species depends on the total number of electron pairs in the valence shell of the central atom, including both bonding... 

Non-oxo pentacoordinated vanadium complexes can adopt the stmcture of a trigonal bipyramid [VF5 in accord with valence shell-electron pair repulsion (VSEPR) theory], or that of a square pyramid [predicted for ( 113)5]. ]... 

The increase of the Be—C bond distance in the complex as well as the relative magnitudes of the < C-Be-C and < N—Be—N angles is easily rationalized in terms of the valence-shell electron-pair repulsion (VSEPR) theory In the complex, repulsion between the electron pairs partly introduced into the formerly empty 2p orbitals of dimethylberyllium and the electron pairs in the Be—C bonds would be expected to weaken the Be—C bonds and push them together. 

The observed variations in the N—C bond distances and < C3—N—C angles are not in agreement with the valence-shell electron-pair repulsion model. This predicts that partial removal of the lone-pair electrons on the nitrogen atom would lead to a shortening of the N—C bonds. It would further predict that < C3—N—C should be greater than tetrahedral in the free donor and decrease on complex formation. 

You recall from Lecture 7 that the VSEPR (valence shell electron pair repulsion) rules predict the shapes of main-element molecules with nice compact reasoning, by simply counting the number of electron pairs, bond pairs and lone pairs around the central atom. In transition metal (TM) complexes, the lone pairs on the TM do not occupy space in the same manner as they do in the case of the main group elements, so what matters for the 3D shape is only the number of bond pairs the TM maintains with the ligands. This number is also known as the coordination number (cn) by the historical referral of Werner to these complexes as coordination complexes. 

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