Document:

<PAGE>

                                                                   Exhibit 10.51

                            TENTH AMENDMENT TO LEASE

      This Amendment made as of the 31st day December, 1996 is by and between
Grove Street Associates of Jersey City Limited Partnership, a New Jersey limited
partnership having an office at c/o Cali Realty Corporation, 11 Commerce Drive,
Cranford, New Jersey 07016 (hereinafter called "Landlord"), and Donaldson,
Lufkin & Jenrette Securities Corporation, a Delaware corporation having an
office at 277 Park Avenue, New York, New York 10172 (hereinafter called
"Tenant").

                                   WITNESSETH

      WHEREAS, Landlord and Tenant have previously entered into a Lease as
described on the attached Schedule #1 covering certain premises in the building
at 1 Pershing Plaza, Jersey City, New Jersey (the "Building"); and

      WHEREAS, Tenant wishes to lease a portion of the space on the floor
designated as 12A of the Building.

      WHEREAS, Landlord and Tenant desire to amend the Lease as set forth
herein.

      NOW, THEREFORE, in consideration of the foregoing, the sum of Ten Dollars
($10.00) and other good and valuable consideration the receipt and sufficiency
of which are hereby acknowledged, Landlord and Tenant do hereby agree to amend
and modify the Lease as follows:

1.    All defined terms used in this Amendment shall have the meanings ascribed
      to them in the Lease unless otherwise defined herein.

2.    The recitals set forth above are incorporated herein by reference.

3.    Landlord hereby leases to Tenant and Tenant hereby hires from Landlord
      approximately 6,507 rentable s.f which represents a portion of the
      rentable area on the floor designated as 12A of the Building (the "12A
      Space") in its "AS-IS" condition as shown cross-hatched on the attached
      Exhibit A.

4.    The term applicable to the 12A Space shall commence on the full execution
      of this Agreement and shall expire on the Expiration Date of the Lease.

5.    The first sentence of Section 1.02 of the Lease is hereby amended to read
      in its entirety as follows:

      "The premises hereby leased to Tenant are described on the attached
      Schedule #2."

6.    In addition to the fixed rent to be paid to Landlord by Tenant for the
      Premises as previously set forth in the Lease, Lessee shall pay Landlord
      fixed rent for the 12A Space as follows:

      (i)   TEN THOUSAND THREE HUNDRED FORTY-SIX AND 13/100 DOLLARS ($10,346.13)
            per month for the period commencing March 1, 1997 and ending July
            13, 1999;

      (ii)  TWELVE THOUSAND FOUR HUNDRED SEVENTEEN AND 53/100 DOLLARS
            ($12,417.53) per month for the period commencing July 14, 1999 to
            July 13, 2004;

      (iii) FOURTEEN THOUSAND NINE HUNDRED ONE AND 03/100 DOLLARS ($14,901.03)
            per month for the period commencing July 14,2004 to July 13, 2009.

      The fixed rent shall be payable in the same manner and under the same
      terms and conditions as the fixed rent currently provided in the Lease.

7.    As of March 1, 1997, Section 5.01(e) of the Lease is amended to provide
      that Tenant's Proportionate Share shall be 66.55% (65.5% +1.05%), and that
      the agreed upon rentable square foot area of the Demised Premises shall be
      deemed to be 413,852 square feet.

8.    As of March 1, 1997, Section 5.07(k) of the Lease is amended to provide
      that Tenant's Operational Proportionate Share shall be 67.28%
      (66.22% +1.06%).

<PAGE>

9.    On or before March 1, 1997, Landlord shall provide Tenant a check in the
      amount of One Hundred Ninety-Five Thousand Two Hundred Ten and 00/100
      Dollars ($195,210.00) towards the cost of improving the Premises (the "12A
      Improvement Allowance"). All work performed in the 12A Space shall be
      governed by the provisions of Article 13 "Tenant's Changes" of the Lease.
      At Landlord's request, Tenant shall furnish Landlord with copies of paid
      invoices related to work paid for with the 12A Improvement Allowance. From
      time to time, upon reasonable notice from Landlord, Tenant shall provide
      Landlord with "as-built" drawings or a "CADD" disk containing the
      "as-built" information with respect to the 12A Space or any other areas of
      the Premises.

10.   The garage parking space allocation applicable to the 12A Space is five
      (5) spaces. At any time during the Term, Tenant may elect to lease all or
      less than all of these spaces by sending written notice to Lessor of its
      intention to lease same. Upon Tenant's election to lease the garage
      parking spaces, Tenant agrees to pay Lessor the monthly rate Lessor is
      then charging other tenants in the Building for each space so leased.

11.   The electric current for the 12A Space shall be supplied by Landlord for
      use by Tenant during the term. Landlord, at Tenant's expense, shall
      install a Building Standard Quad Logic check meter to measure the
      electrical consumption of the 12A Space. Tenant shall pay to Landlord,
      monthly, as Additional Rent, the cost of providing the electricity
      (including the cost of the meter reading service) to the 12A Space at
      terms, classifications and rates normally charged to the Building by the
      public utilities corporation servicing that part of Jersey City where the
      Building is located. All computations shall be made on the basis of
      Tenant's metered usage.

12.   With respect to the Tenth Lease Amendment, Article 32 "Broker" is hereby
      deleted and the following shall apply to the leasing of the 12A Space:
      "Landlord and Tenant represent and warrant to each other that no broker
      brought about this transaction, and the parties agree to indemnify and
      hold each other harmless from any and all claims of any broker (claiming
      to have dealt with the indemnifying party) arising out of or in connection
      with the negotiations of or entering into of this Amendment by Tenant and
      Landlord. Landlord represents that there is no commission due to Cushman
      and Wakefield in connection with this transaction pursuant to any prior
      agreement between Landlord and Cushman and Wakefield.

13.   Tenant represents and warrants that this Lease and the undersigned's
      execution of same has been duly authorized and approved by the
      corporation's Board of Directors. The undersigned officer of the
      corporation represents and warrants he is an officer of the corporation
      with authority to execute this Lease on behalf of the corporation.
      Landlord represents and warrants that this Amendment and the undersigned's
      execution of same has been duly authorized and approved by all necessary
      partnership and corporate actions. The undersigned officer of the
      corporate general partner of Landlord represents and warrants that he is
      an officer of the corporation with authority to execute this Amendment on
      behalf of the corporate general partner of Landlord, and that the
      corporate general partner of Landlord is authorized to execute this
      Amendment on behalf of Landlord.

      EXCEPT as modified herein, the Lease dated July 1, 1987 and all amendments
and sideletters applicable thereto covering the Premises shall remain in full
force and effect as if the same had been set forth in full herein and Tenant and
Landlord hereby ratify and confirm all of the terms and conditions thereof.
Tenant acknowledges that it has no offsets, defenses or counterclaims to its
obligations under the Lease as amended hereby.

      THIS Amendment shall be binding upon and inure to the benefit of the
parties hereto and their respective legal representatives, successors and
permitted assigns.

GROVE STREET ASSOCIATES OF                  DONALDSON, LUFKIN & JENRETTE
JERSEY CITY LIMITED PARTNERSHIP             SECURITIES CORPORATION
(Landlord)                                  (Tenant)

By: Cali Sub IV, Inc.
    General Managing Partner

By: /s/ James G. Nugent                     By: /s/ Robert A. Yurman
    -----------------------------------         -------------------------------
    James G. Nugent                             Robert A. Yurman, Vice President
    Vice President - Leasing                    Director of Administration

<PAGE>

                                   EXHIBIT A

                              [FLOOR PLAN OMITTED]

<PAGE>

                     SCHEDULE #1 TO LEASE DATED JULY 1, 1987

LANDLORD: Grove Street Associates of Jersey City Limited Partnership

TENANT: Donaldson, Lufkin & Jenrette Securities Corporation

                            DESCRIPTION OF THE LEASE

DATE OF DOCUMENT         DOCUMENT

July 1, 1987             Lease
July 1, 1987             First Amendment of the Lease
July 1, 1987             Side Letter Amendment
June 19, 1989            Side Letter Agreement
March 12, 1992           Second Amendment to Lease
December 27, 1992        Third Amendment to Lease
September 29, 1993       Side Letter Agreement
December 23, 1993        Fourth Amendment to Lease
May 1, 1994              Fifth Amendment to Lease
March 9, 1995            Sixth Amendment to Lease
June 16, 1995            Seventh Amendment to Lease
April 4, 1996            Eighth Amendment to Lease
April 4, 1996            Ninth Amendment to Lease

                                      END

<PAGE>

                    SCHEDULE #2 to Lease dated July 1, 1987

      Landlord: Grove Street Associates of Jersey City Limited Partnership

          Tenant: Donaldson, Lufkin & Jenrette Securities Corporation

                       Description of the LEASED PREMISES

                                        OFFICE          STORAGE       TEMPORARY
       FLOOR(S)                         SPACE            SPACE          SPACE
       --------                         -----            -----          -----

         2-9                           256,925
         10                             35,820
                                       -------
        2-10                           292,745
        Lobby                            4,800
                                       -------
Lobby, 2-10                            297,545
         11                             36,600
                                       -------
Lobby, 2-11                            334,145
Storage(kitchen)                                          8,194
Storage Space #1                                          1,800
Storage Space #2                                          3,747
Temporary Space-12th Floor                                              4,809
Storage-Temporary #1                                                      794
Storage-Temporary #2                                                      112
        14&15                           73,200
                                       -------
Subtotal                               407,345
         12A                             6,507                0             0
                                       -------           ------         -----
TOTAL S.F. AREAS                       413,852           13,741         5,715
                                       =======           ======         =====

<PAGE>

                   TENANT IMPROVEMENT - WORK LETTER ESTIMATES

                         Description: DLJ 3/1/97 6 50 7
                                      ----------------------------
             (30 characters) Tenant Name/Lease Date/S.F. (Rentable)

Property Manager:       Rob Vicci
                        --------------
Date:                   1/31/97
                        --------------
Property:               O.C.
                        --------------
Tenant ID:              PE
                        --------------
Phase (YR):             97
                        --------------
Bldg (Job #):           99
                        --------------
(start @ 99)

                                                               Budget
GL#          Description                                       Estimated Cost)
---          -----------                                       -----------------
5530         PARTITIONS
                                                               -----------------
5540         DOORS AND HARDWARE
                                                               -----------------
5533         ELECTRICAL
                                                               -----------------
5535         VENTILATION
                                                               -----------------
5565         PAINTING AND VINYL WALL COVER
                                                               -----------------
5564         FLOOR AND CARPET
                                                               -----------------
5574         FIRE SPRINKLER
                                                               -----------------
5561         CABINETS
                                                               -----------------
5532         PLUMBING
                                                               -----------------
5510         GENERAL CONDITIONS
                                                               -----------------
5524         MISCELLANEOUS
                                                      TOTAL    $l95,210.00
                                                               -----------------

Work Completed                       Tenant to complete

All bills submitted         Dated:   1/31/97<PAGE>

                                                                   Exhibit 10.52

                          ELEVENTH AMENDMENT TO LEASE

      This Amendment made as of the 7th day of February, 1997 is by and between
Grove Street Associates of Jersey City Limited Partnership, a New Jersey limited
partnership having an office at c/o Cali Realty Corporation, 11 Commerce Drive,
Cranford, New Jersey 07016 (hereinafter called "Landlord"), and Donaldson,
Lufkin & Jenrette Securities Corporation, a Delaware corporation having an
office at 277 Park Avenue, New York, New York 10172 (hereinafter called
"Tenant").

                                   WITNESSETH

      WHEREAS, Landlord and Tenant have previously entered into a Lease as
described on the attached Schedule #1 covering certain premises in the building
at 1 Pershing Plaza, Jersey City, New Jersey (the "Building"); and

      WHEREAS, Tenant wishes to use a portion of the space on the top floor of
the Building's parking garage for the installation of dry coolers.

      WHEREAS, Landlord and Tenant desire to amend the Lease as set forth
herein.

      NOW, THEREFORE, in consideration of the foregoing, the sum of Ten Dollars
($10.00) and other good and valuable consideration the receipt and sufficiency
of which are hereby acknowledged, Landlord and Tenant do hereby agree to amend
and modify the Lease as follows:

1.    All defined terms used in this Amendment shall have the meanings ascribed
      to them in the Lease unless otherwise defined herein.

2.    The recitals set forth above are incorporated herein by reference.

3.    Subject to the terms of this Agreement, Landlord hereby grants Tenant the
      right to install dry coolers and associated screening applicable thereto
      (the "Installation") on the roof of the parking garage of the Building.

4.    The Installation shall:

      a)    be located on the top floor of the parking garage adjacent to the
            Building as shown cross-hatched on Exhibit A attached hereto;

      b)    be governed by the provisions of Article 13 "Tenants" Changes of the
            Lease without regard to the cost of the Installation except that
            Article 13.01(b) of the Lease shall be waived as it applies to the
            installation.

      (c)   include a metal louvered screening system around the dry coolers no
            less than 5' 6" in height.

      (d)   provide door access to it only from the south side of same.

      (e)   at Landlord's option, shall be removed by Tenant at Tenant's expense
            upon the Expiration Date or sooner termination of the Lease and
            Tenant shall fully repair any damage to the Building and parking
            garage resulting from such removal (this obligation shall survive
            the expiration or termination of the Lease).

5.    Upon completion of the Installation, the number of Free Spaces as defined
      in Article 45.03 shall be reduced to thirty-seven (37) from fifty (50).
      The parties hereto acknowledge ten (10) Free Spaces have been previously
      deducted from the original total when Tenant elected to use same for the
      installation of the sixth floor cooling towers, the fifth floor sump tanks
      and the ground floor fuel oil storage tank. In the event the Installation
      is removed by Tenant prior to the Expiration Date or sooner termination of
      the Lease, the Free Spaces shall be increased to forty (40) from
      thirty-seven (37).

                                       1
<PAGE>

6.    Tenant represents and warrants that this Lease and the undersigned's
      execution of same has been duly authorized and approved by the
      corporation's Board of Directors. The undersigned officer of the
      corporation represents and warrants he is an officer of the corporation
      with authority to execute this Lease on behalf of the corporation.
      Landlord represents and warrants that this Amendment and the undersigned's
      execution of same has been duly authorized and approved by all necessary
      partnership and corporate actions. The undersigned officer of the
      corporate general partner of Landlord represents and warrants that he is
      an officer of the corporation with authority to execute this Amendment on
      behalf of the corporate general partner of Landlord, and that the
      corporate general partner of Landlord is authorized to execute this
      Amendment on behalf of Landlord.

      EXCEPT as modified herein, the Lease dated July 1, 1987 and all amendments
and sideletters applicable thereto covering the Premises shall remain in full
force and effect as if the same had been set forth in full herein and Tenant and
Landlord hereby ratify and confirm all of the terms and conditions thereof.
Tenant acknowledges that it has no offsets, defenses or counterclaims to its
obligations under the Lease as amended hereby.

      THIS Amendment shall be binding upon and inure to the benefit of the
parties hereto and their respective legal representatives, successors and
permitted assigns.

GROVE STREET ASSOCIATES OF                DONALDSON, LUFKIN & JENRETTE
JERSEY CITY LIMITED PARTNERSHIP,          SECURITIES CORPORATION,
(Landlord)                                (Tenant)

By:   Cali Sub IV, Inc.
      General Managing Partner

By:   /s/ James G. Nugent                 By:   /s/ Robert A. Yurman
      ----------------------------              --------------------------------
      James G. Nugent                           Robert A. Yurman, Vice President
      Vice President - Leasing                  Director of Administration

                                       2
<PAGE>

                     SCHEDULE #1 TO LEASE DATED JULY 1, 1987

LANDLORD: Grove Street Associates of Jersey City Limited Partnership

TENANT: Donaldson, Lufkin & Jenrette Securities Corporation

                            DESCRIPTION OF THE LEASE

DATE OF DOCUMENT                       DOCUMENT
----------------                       --------

July 1, 1987                           Lease
July 1, 1987                           First Amendment of the Lease
July 1, 1987                           Side Letter Amendment
June 19, 1989                          Side Letter Agreement
March 12, 1992                         Second Amendment to Lease
December 27, 1992                      Third Amendment to Lease
September 29, 1993                     Side Letter Agreement
December 23, 1993                      Fourth Amendment to Lease
May 1, 1994                            Fifth Amendment to Lease
March 9, 1995                          Sixth Amendment to Lease
June 16, 1995                          Seventh Amendment to Lease
April 4, 1996                          Eighth Amendment to Lease
April 4, 1996                          Ninth Amendment to Lease
December 31, 1996                      Tenth Amendment to Lease

                                                END

<PAGE>

                    SCHEDULE #2 to Lease dated July 1, 1987

      Landlord: Grove Street Associates of Jersey City Limited Partnership

          Tenant: Donaldson, Lufkin & Jenrette Securities Corporation

                       Description of the LEASED PREMISES

                                        OFFICE          STORAGE       TEMPORARY
       FLOOR(S)                         SPACE            SPACE          SPACE
       --------                         -----            -----          -----

         2-9                           256,925
         10                             35,820
                                       -------
        2-10                           292,745
        Lobby                            4,800
                                       -------
Lobby, 2-10                            297,545
         11                             36,600
                                       -------
Lobby, 2-11                            334,145
Storage(kitchen)                                          8,194
Storage Space #1                                          1,800
Storage Space #2                                          3,747
Temporary Space-12th Floor                                              4,809
Storage-Temporary #1                                                      794
Storage-Temporary #2                                                      112
        14&15                           73,200
                                       -------
Subtotal                               407,345
         12A                             6,507                0             0
                                       -------           ------         -----
TOTAL S.F. AREAS                       413,852           13,741         5,715
                                       =======           ======         =====

<PAGE>

                                    EXHIBIT A

                              [FLOOR PLAN OMITTED]

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