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Apart from hydrogen which forms a duet, which element's don't observe the octet rule?

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Octet rule is more advisory than a rule; It's usually obeyed by the main group elements. Exception are paramagnetic compounds (obvious reason); first for elements in periodic system (He-configuration), 3-valent boron compounds; depending on counting system boron clusters (f.e. dodecaborane with 6-coordianted boron); transition metal complexes (these usually follow 18e rule, however there are plenty of examples with 14, 16 or even 20 electron complexes).

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  • $\begingroup$ It would be nice if you give some examples for TM-complexes not obeying 18e rule. $\endgroup$ – tatus2 Jun 30 '13 at 8:18
  • $\begingroup$ Enough that you look at the metallocenes. 15e: VCp2 (Cp=C5H5-); 16e: CrCp2; 17e: MnCp'2 (Cp'=C5H4Me-); 19e: CoCp2; 20e: NiCp2. Most of them are commercial products. $\endgroup$ – Kris_R Jun 30 '13 at 15:00

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