Multiplicity character

In each case the P—O bonds have some multiple character. Phosphinic acid is a moderately strong monobasic acid. On heating the acid and its salts they disproportionate evolving phosphine ... 

Interaction of the iron metal atoms with thiophenes (thiophene, 2-methyl-, and 2,5-dimethylthiophene) in the vapor phase at 77 K with subsequent heating in a carbon monoxide atmosphere also leads to the formation of ferrole 83 [76JOM(l 18)37, 77CJC3509]. The iron cyclopentadienyl ring is planar and all the bonds have multiple character. 

There is considerable multiple character in the carbene carbon-X(Y) bonds. An internal comparison of carbon-X(Y) bond lengths in each of the structures tabulated reveals a substantial shortening of carbene car-bon-X(Y) relative to other carbon-X(Y) bonds in the molecule. Carbene carbon-nitrogen distances are found to be shorter in the mono-N-substituted carbenes than in the bis-N-substituted compounds. 

The major metric difference is in the WC (carbene) and ReC alkyl) bonds, the former being shorter due to its multiple character. The similarity in CO distances implies negligible 0 - C multiple bonding in the W/Zr compound. This is a consequence of 0 - Zr ir donation in both compounds (compare the Zr-0 bond lengths), which represents the dominant utilization of the oxygen lone pairs. 

The bond length data, although not satisfactory, do suggest that the P—C bond has multiple character with a bond order of about 2. Other physical measurements which support this and provide further insight are detailed below. Note that many of the original studies which reported these data discuss them in terms of dn-pn overlap on the basis that it is evidence for such overlap. Since the d orbital concept is now discarded, these discussions are obsolete, but the experimental results still give useful information. 

The consequences of vacant d orbitals are also evident on comparing the amine oxides, R3NO, with R3PO or R3AsO. In the V-oxide the electronic structure can be represented by the single canonical structure R3N+—O-, whereas for the others the bonds to oxygen have multiple character and are represented as resonance hybrids ... 

Fischer recognized the first carbene complexes in 1964. They were formed by the attack of an alkyllithium on a metal carbonyl followed by methylation (equations 1 and 2). Resonance form (2), considered as the dominant one in the heteroatom stabilized Fischer carbenes, shows the multiple character of this carbon-heteroatom bond. This effect is responsible for the restricted rotation often observed for this bond in nuclear magnetic resonance (NMR) studies. For example cis and trans isomers (6) and (7) of methoxymethyl carbenes rapidly interconvert at room temperature, but can be frozen out in the proton NMR at -40 °C. By contrast, the M-C bond is close to single and often rotates freely. 

B. Cleaning up keyboard signals and preventing multiple characters from a... 

The X-ray crystal structures of many oxovanadium(IV) complexes have been determined. In general, the V=0 bond is shorter than most other V-0 bond lengths, ranging between 1.57 and 1.68 A, which is in keeping with the above view of the multiple characters of this bond. The most common... 

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