Twisted dative bonds

It is interesting to consider also the triplet states. We have shown that T, lies close to Sj for TICT states of a simple dative bond but locally excited triplets certainly lie well above locally excited singlets. Therefore, a very strong donor-acceptor combination in the case of the complex dative bond will be needed in order to obtain minimum in T, for twisted geometries. 

The carbene-pnictinidene (93-97) adducts can be represented by two extreme canonical forms (98 and 99). The canonical form (99) represents a conventional phosphaalkene with a C=P double bond, whereas form (98) corresponds to a phosphinidene adduct of a carbene and features a C-P dative bond order of one. Structural data for compounds (93-97) showed that the E-C(Carbene) bond is 4% longer than typical single bonds and that the E-C(R") moiety is twisted out of the carbene plane by 26-46°. These data, along with the high-held chemical shifts of (93,94) and (96), indicate the predominance of structure (98). 

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