P1: JLS

Structural Chemistry (STUC) pp1128-stuc-481012 February 27, 2004 3:27 Style file version Nov. 07, 2000

Structural Chemistry, Vol. 15, No. 3, June 2004 (C© 2004)

Geometrical Changes and Their Energies in theFormation of Donor–Acceptor Complexes

Vikt oria Horv ath1,2 and Istvan Hargittai 1,2

Received June 16, 2003; accepted September 12, 2003

We have mapped the energy demands of the geometrical changes in donor–acceptor complexesBH3·NH3 and AlCl3·NH3 and in the course of their formation from their monomers. We have variedthe individual geometrical parameters systematically and performedab initio quantum chemicalcalculations for these structures. We investigated the energy requirements to change bond lengths andbond angles in both the monomers and complexes and the angles of torsion in the complexes. Thechanges of bond lengths require more energy in the monomers than in the complexes. The energiesto change the acceptor bond angles in the monomers are markedly higher than in the complexes. Thechanges in the geometrical parameters during the complexation process are more moderate in donorsthan in acceptors, in agreement with prior experimental observations. The geometry versus energyvariations related to the process of complexation are in agreement with the notion of relative rigidityof the donor parts and the more compliant nature of the acceptor parts as well as with the notion ofcompeting effects in the structures of the complexes.

KEY WORDS: Donor acceptor complexes; theoretical study; BH3·NH3; AlCl3·NH3.

INTRODUCTION

We mapped the energy requirements of the geomet-rical changes in the course of the formation of donor–acceptor complexes BH3·NH3 and AlCl3·NH3 from theirrespective monomers. We than carried out systematicvariations of the individual geometrical parameters,as Hargittai and Levy [1] had done for alkanes.

COMPUTATIONAL DETAILS

Our calculations were based on the equilibrium ge-ometries calculated by us earlier at the MP2(fc) level witha 6-311+G(2df,p) basis set [2]. Based on these geome-tries, single-point calculations were performed with theabove method and basis set, with the relevant geometrical

1Institute of General and Analytical Chemistry, Budapest Universityof Technology and Economics, H-1111 Budapest, Szt. Gell´ert ter 4,Hungary.

2Structural Chemistry Research Group of the Hungarian Academy ofSciences at E¨otvos University, Budapest, Hungary.

3To whom correspondence should be addressed; e-mail: hargittai.istvan@tki.aak.bme.hu

parameters being varied systematically, while all other pa-rameters were kept at the optimized values. Calculationswere performed using the GAUSSIAN98 [3] programpackage.

Variations of three types of geometrical parametersin the monomers and complexes, bond lengths, bondangles, and torsion angles (for complexes only), havebeen investigated. The bond lengths were altered up to±0.04A, as compared with the equilibrium bond length,by a 0.01A step. The donor bond angles were changedby a 0.5◦ step up to±2.5◦. For the acceptor monomers,in which the equilibrium angles are 120◦, the angles wereonly decreased; this decrease was extended for a largerange. Similar calculations were done for the acceptorparts of the complexes. The angles of torsion in thecomplexes were altered with up to±5◦, by a 1◦ step, ascompared with the 60◦ equilibrium angle. The calculatedenergy profiles are shown in Fig. 1.

RESULTS AND DISCUSSION

A number of observations could be made for which,however, a caveat must be issued. These observations are

2331040-0400/04/0600-0233/0C© 2004 Plenum Publishing Corporation

P1: JLS

Structural Chemistry (STUC) pp1128-stuc-481012 February 27, 2004 3:27 Style file version Nov. 07, 2000

234 Horvath and Hargittai

Fig. 1. Energy profiles of changes in the individual parameters.

valid to the extent to which the assumption of the sep-arability of the parameter changes is applicable. It is anapproximation whose validity increases as the changesexamined diminish.

The changes of the bond lengths require somewhatmore energy in the monomers than in the analogouscomplexes, this difference not being significant for theN H bonds. In the acceptors, more energy is needed to

P1: JLS

Structural Chemistry (STUC) pp1128-stuc-481012 February 27, 2004 3:27 Style file version Nov. 07, 2000

Geometrical and Energetical Changes in the Formation of Donor–Acceptor Complexes 235

stretch the B H bonds than the Al Cl bonds, both inthe monomers and in the complexes. The energy require-ment is more than twice for the changes in the NH bondlengths than for similar bond length changes in acceptors.

There is a higher energy requirement to initiallychange the acceptor angles. The acceptor monomerappears to be more rigid and less compliant than theanalogous moiety in the complex. Therefore, we haveinvestigated their angles in a wider range, both in themonomers and in the complexes than the donor angle, asit is shown in Fig. 1. The highly symmetrical (D3h) ac-ceptor monomer appears to resist the lowering of its sym-metry. On the other hand, the donor monomer initially hasa lower symmetry (C3v) and it appears to be more com-pliant. Less energy is required to change its angle thanto change the analogous angles in the complexes. Thechanges in the acceptor angles in the complexes requireless energy than those in the donor angles. A slight dif-ference is discernable between the BH3 and AlCl3 parts,the former appearing a little less compliant than the latter.

Changes in the angles of torsion require more energyin the BH3·NH3 complex than in the AlCl3·NH3 com-plex. A major factor in making torsion less hindered in theAlCl3·NH3 complex may be its longer (1.999A) donor-acceptor distance as compared with the analogous distance(1.650A) in the BH3·NH3 complex. Thus, the latter ap-pears to be a more crowded molecule than the former.

For better comparison, the average energy amountsassociated with characteristic changes in the individualparameters are listed for selected points in Table I. Theaverages refer to changes in the parameters in both di-rections, where applicable, ignoring the asymmetry of thevariations between, for example, bond stretching and bondcompression since these asymmetries are small for theranges examined in this table.

It is of particular interest to see the geometricalchanges and associated energy requirements for the pro-

Table I. Average Energy Requirements Associated with GeometricalParameter Changes in the Monomers and Complexes

1parameter 1E (kJ-mol−1)

BH3 BH3·NH3

B H ±0.02A 1.5 1.3H B H −2◦; ±2◦ 16.9 1.9

AlCl3 AlCl3·NH3

Al Cl ±0.02A 1.2 1.0Cl Al Cl −2◦; ±2◦ 14.5 3.8

NH3

N H ±0.02A 2.6 2.6 2.5H N H ±2◦ 0.6 1.0 0.9H B N H ±5◦ 0.2Cl Al N H ±5◦ 0.04

Table II. Changes in Geometric Parameters during ComplexFormation and Associated Energy Requirements

Parameter Monomer Complex1parameter1E (kJ-mol)−1

Bond lengths,AB H 1.189 1.208 0.02 1.4Al Cl 2.073 2.114 0.04 4.4N H(BH3·NH3) 1.013 1.017 0.003 <0.1N H(AlCl3·NH3) 1.013 1.019 0.006 0.2Angles (deg.)H B H 120.0 113.6 −6.4 57.6Cl Al Cl 120.0 116.5 −3.5 25.7H N H(BH3·NH3) 107.5 108.0 0.5 <0.1H N H(AlCl3·NH3) 107.5 107.8 0.3 <0.1

cess of complexation as listed in Table II. All changes inthe donor part are considerably smaller than those in theacceptor parts. Hargittai and Hargittai [4,5] have noted be-fore the impact of the lone pair becoming a fourth bondingdomain for the acceptor during complexation and the newnonbonded interactions appearing as repulsions as a resultof the donor ligands getting nearer to the acceptor ligandsduring the same process. For the acceptor, both effects en-hance the geometric deformation. For the donor, its lonepair becomes a bonding domain and the new nonbondedinteractions are counteracting and, thus, somewhat bal-ancing each other, resulting in muted changes.

Complex formation invariably causes bond elonga-tions, the largest effect occurring in AlCl3, which may bethe result of this bond being the most compliant of all thebonds figuring in the systems investigated in this study. Incontrast to the considerable angular closure of the accep-tor parts, the H N H angles in the donors slightly openas a result of the complex formation. Accordingly, thehydrogen ligands are getting slightly nearer to the accep-tor parts. However, angular openings provide a mitigatingeffect in the steric repulsions between the ligands of thecomplexing moieties.

ACKNOWLEDGMENT

The Ministry of Education of Hungary (FKFP0364/1999) supported this research. We are very grate-ful to Dr. Attila Kovacs for valuable help and discussions.

REFERENCES

1. Hargittai, I. and Levy, J. B.Struct. Chem.1999,10,387.2. Horvath, V.; Kovacs, A. and Hargittai, I.J. Phys. Chem.2003,107,

1197.3. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,

M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, Jr., J. A.;

P1: JLS

Structural Chemistry (STUC) pp1128-stuc-481012 February 27, 2004 3:27 Style file version Nov. 07, 2000

236 Horvath and Hargittai

Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;Morokuma, K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko,A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox,D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;

Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.;Replogle E. S. and Pople, J. A. GAUSSIAN98; Gaussian Inc.:Pittsburgh, Pennsywania, 1998.

4. Hargittai, M. and Hargittai, I.The Molecular Geometries of Co-ordination Compounds in the Vapour Phase; Akademiai Kiado:Budapest, 1977; pp. 68–71.

5. Hargittai, M. and Hargittai, I.J. Mol. Struct.1977,39,79.