The molecular structures and thermodynamic functions of seven dialkyl zinc compounds, R 2 Zn, R = Me, Et, i-Pr, t-Bu, n-Pr, neopentyl and the silaneopentyl Me 3 SiCH 2 , of the parent hydrocarbons RH and of the radicals R have been determined by density functional theory calculations at the B3LYP/SDD level. The molecular structures of the i-Pr, t-Bu, neo-Pe and Me 3 SiCH 2 derivatives have been determined by gas electron diffraction. Me 2 Zn, Et 2 Zn, t-Bu 2 Zn and neo-Pe 2 Zn have been studied by photoelectron spectroscopy and the ionisation energies calculated. Both experimental and calculated Zn-C bond distances were found to increase in the orderCalculated mean bond rupture enthalpies indicate that the strength of the Zn-C bonds decrease in the same order, viz.Both bond lengths and bond strengths were found to be strongly correlated with the inductive Taft constant I, indicating that the bond strength increases and the bond length decreases with increasing electron withdrawing power of the alkyl group. Evidence from the literature indicates that bond strengths and bond lengths in homoleptic alkyl derivatives of the main group metals in Groups 12, 13 and 14 and of the transition elements in Group 4 vary in the same manner.D a l t o n T r a n s . ,

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The molecular structures of dimethylamino[(dimethylboryl)methylamino]methylborane, Me2NBMeNMeBMe2 (1) and 1,1-bis(dimethylboryl)-2,2-dimethylhydrazine, (Me2B)2NNMe2 (2) have been determined by gas electron diffraction (GED), density functional theory calculations at the B3PW91/6-311++G** level and ab initio calculations at the MP2/6-311++G** level. 1 adopts an open structure similar to that of the isoelectronic hydrocarbon molecule permethylbutadiene; the central B-N bond distance at 148.0/149.3(7) pm (MP2/GED) corresponds to a single covalent N--B bond distance, the two terminal distances, 140.9/140.5(4) pm and 141.8/141.3(4) pm, correspond to the distance between N and B atoms joined by a covalent sigma-bond and a dative pi-bond. A closed form where the establishment of a dative bond between the terminal N and B atoms has led to the formation of a four-membered ring also corresponds to a minimum on the potential energy surface, but the energy is calculated to be 14.3 kJ mol(-1) higher at the MP2 level. This structure is also incompatible with the GED data. 2 adopts a structure in which a dative sigma-bond between the dimethylamino N atom and one of the boron atoms has led to the formation of a three-membered N(2)B ring. The dative sigma-bond distance is 165.5/164.0(13) pm, the two other bond distances in the ring are N-B=150.6/148.9(9) pm corresponding to a covalent sigma-bond and N-N=145.1/145.4(3) pm. The terminal B--N distance 139.6/138.9(9) pm is consistent with a covalent sigma-bond augmented by a dative pi-bond. An open Y-shaped structure also corresponds to a minimum on the potential energy surface, but the energy is 18.7 kJ mol(-1) higher (MP2) and it is incompatible with the GED data.

The nature of the two-center, two-electron (2c,2e) dative π-bonding system in aminoboranes has been explored by optimizing the equilibrium structures of R 2 NBR′ 2 (R, R′ ) H or Me) by DFT calculations at the B3PW91/6-311++G** level. The π-bond rupture energies were determined by optimizing models in which the relative orientation of the R 2 N and BR′ 2 fragments was fixed in such a manner that the lone-pair atomic orbital on N was orthogonal to the vacant 2p-orbital on the B atom. Similar calculations were carried out on the (3c,2e) dative π-bonding systems of bisborylamines, RN(BR′ 2 ) 2 ; the (3c,4e) π-system of bisaminoboranes, RB(NR′ 2 ) 2 ; the trigonal (4c,2e) π-bonding systems of trisborylamines, N(BR 2 ) 3 , and the (4c,6e) π-systems of trisaminoboranes, B(NR 2 ) 3 . The structures of HN(BMe 2 ) 2 , MeN-(BMe 2 ) 2 , and MeB(NMe 2 ) 2 determined by gas electron diffraction were in good agreement with those determined by calculations. The mean N-B π-bond rupture energies, 〈D π 〉, in the prototypical compounds H 2 NBH 2 , HN(BH 2 ) 2 , HB(NH 2 ) 2 , N(BH 2 ) 3 , and B(NH 2 ) 3 were found to depend not on the number of π-electrons, but on the number of centers. The mean π-bond rupture energies of the prototypical 2c, 3c, and 4c compounds were found to vary in the order 4.0:3.0:2.0. When H atoms at an acceptor atom (B) are replaced by more electron releasing methyl groups, 〈D π 〉 is significantly reduced due to a synergetic combination of destabilizing inductive effects and steric strain. When H atoms at the donor atom (N) are replaced by Me groups, the effect on 〈D π 〉 is determined by the balance of stabilizing inductive and destabilizing steric effects. The N-B bond distances in the 14 molecules tend to decrease with increasing mean π-bond rupture energy. Linear correlation analysis yields R(N-B) ) 149.3 pm -(0.075 pm mol kJ -1 ) 〈D π 〉 and a correlation coefficient of F ) 0.97.

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