Valence angles, abnormal

Rule 8-Abnormal valence angles may be found if they increase HOMO-LUMO interactions between molecular fragments... 

The family of complexes of nucleophilic carbenes or alkylidenes is essentially composed with d complexes. For instance, the complexes [M(CH2t-Bu)3(=CHr-Bu)], M = Nb or Ta formally only have 10 valence electrons. The X-ray diffraction structures of Schrock s alkylidene complexes that have less than 18 valence electrons show that the M-C-H angle is weak and that the C-C-H angle is abnormally high, i.e. there is an agostic M=C-H bond that increases the NVE by two units ... 

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